Updated
Short answer: No — and the reason isn’t that ShipStation left a feature out. It’s that “flat rate vs. standard rate” isn’t a rate shopping question at all. It’s a packing question wearing a rate shopping costume, and rate shoppers take the package as an input.
This is the most expensive misunderstanding in ShipStation rate optimization, and it’s worth understanding precisely, because it determines whether the money you’re leaving on the table is findable with a setting or not findable at all without a different kind of tool.
This is the deep dive on the first lever in How to Reduce Shipping Costs in ShipStation. If you want the full list of where the savings are, start there. If you want to know exactly why this particular one can’t be configured away, stay here.
Credit where it’s due: Rate Shopper is a real feature that does real work. It’s not the static-rules approximation people used to build by hand.
You create a Rate Shopper rule in Settings → Shipping → Rate Shopper. You pick a strategy — Cheapest, Fastest, or Best Value — or, on higher plans, build a custom configuration. You define which carrier/service combinations are eligible. ShipStation pulls live rates for those services and applies the winner.
You can apply it manually per shipment, or automatically via an automation rule using the Set Rate Shopper action. It has some genuinely thoughtful touches:
Availability, as of this writing: US, UK, Canada, Australia, and New Zealand accounts only. It’s plan-gated, and custom Rate Shopper configurations sit behind a higher tier than the three default strategies. That’s a solid feature. If you’re currently picking services with static automation rules, turn on Rate Shopper before reading the rest of this.
Rate Shopper optimizes the service for a package you have already decided on.
ShipStation says so directly. From the notes at the end of their Rate Shopper documentation:
Rate Shopper only compares services. It does not compare different package types.
That single sentence is the whole of what follows. A comparison that does not consider package types cannot consider a carrier program whose entire premise is a particular package type.
You can see this in ShipStation’s own setup instructions. Their documented example for a Rate Shopper automation rule requires two rules in a specific order: a rule that sets package dimensions first, then the Rate Shopper rule. Their words: “The dimensions rule must come first to ensure that all orders generate the appropriate rate.”
The ordering requirement says the same thing from the other end. Neither is an implementation detail. Both are the architecture, stated out loud. The package is an input to rating. Rating happens downstream of packing. The box is already decided by the time any rate gets fetched.
Which is completely fine — until the box and the rate stop being independent decisions.
Carrier flat-rate programs — USPS Priority Mail Flat Rate, FedEx One Rate, and their equivalents — work on a specific trade: you use our box, we charge one price regardless of weight or zone.
That trade creates a dependency that runs backwards through the normal sequence:
Each answer is an input to the other. That’s a joint optimization, and you can’t solve a joint optimization by doing one step and then the other. Do packing first and you’ve eliminated flat rate before you ever priced it — you packed into your own 12×9×4 and the FedEx One Rate small box was never on the table. Do rating first and you have nothing to rate, because there’s no package yet.
The only way to get the right answer is to price the order against your standard rates and against each flat-rate option’s box constraints at the same time, and take whichever comes out cheaper. That’s one decision, not two.
Any system that treats packing and rating as separate sequential steps — which is nearly all of them, including ShipStation — will systematically miss exactly the cases where switching boxes would have saved money. Not randomly. Systematically, in one direction, on the same order profiles every time. Heavy items going to distant zones, which is precisely where flat rate pays and where standard rates hurt most.
You will never see these misses. The rate shopper reports a win every single time, because it correctly found the cheapest service for the box you gave it. The savings it didn’t find aren’t in any report. There’s no line item for a counterfactual.
Say you’re shipping 6 lbs from Ohio to California — zone 8, heavy, far.
What Rate Shopper does: you’ve set dimensions to your standard 12×9×6 carton. Rate Shopper prices UPS Ground, USPS Ground Advantage, FedEx Home Delivery against that box, and picks the cheapest. It reports a saving versus your default service. Everyone’s happy.
What it didn’t consider: those 6 lbs might fit in a USPS Priority Mail Flat Rate box, where zone 8 and 6 lbs cost exactly the same as zone 1 and 1 lb. On this specific order — heavy, far — flat rate can be dramatically cheaper. But it was never eligible, because you’d already committed to a box that wasn’t theirs.
To catch it, the system would have to ask: do these items physically fit in the flat rate medium box, and if so, what’s that price versus the best standard rate in my own carton? That question requires knowing the dimensions of every item in the order and solving a 3D fit problem — before rating, in order to rate. It’s cartonization and rating as a single operation.
The savings are largest exactly where the sequential approach has least to work with, and the orders where it matters most are the heavy multi-item ones — which, per how SKU criteria evaluate in the rules engine, are also the orders where item-level rules don’t apply.
A few adjacent things to verify against your own account rather than take on faith:
Custom package types in Rate Shopper are regional. ShipStation documents that “you can include a Custom Package Type when you set up Rate Shopper automation rules if your account is based in Australia or New Zealand”, with a defined set of scenarios for custom packages competing against standard packages. If your account is elsewhere, check whether your own custom packaging can participate in a Rate Shopper comparison at all — it changes the shape of what’s even possible.
Not every service is rate-shoppable. Rate Shopper compares services you pre-selected from what’s eligible. Services that sit outside the eligible set can’t win no matter how cheap they are. Historically this has included some consolidator and marketplace-purchased services. Check your own eligible service list rather than assuming.
Bulk edits and Rate Shopper don’t mix cleanly. ShipStation states that “bulk updates to package type and confirmation type will not apply to orders with Rate Shopper” — a real operational wrinkle if your team fixes things in bulk from the grid.
Pre-selection caps the upside. Rate Shopper can only choose from services you put in the rule. If you built an express rule containing only 2-day guaranteed services, an economy service that would have arrived in time anyway can’t be picked. Delivery time constraints help here — but only within the eligible set.
Four honest options.
Do nothing, deliberately. If your products are homogeneous, your boxes are few, and your zones are close, flat rate probably rarely wins and the money you’re missing is small. Turn on Rate Shopper, set it to Cheapest with a sensible service preference, and stop thinking about it. This is the right answer for more brands than vendors like to admit.
Handle it manually on the orders that matter. If you can identify the profile where flat rate tends to win — over N pounds, beyond zone N — pull those orders and have a person check them in the rate calculator. At low volume this is genuinely cheaper than software.
Audit, then write a rule for that profile. This sits between doing nothing and solving it properly, and it’s the option most often left out of this conversation. You can’t make Rate Shopper compare the boxes. You can find the profile where flat rate reliably wins and have an automation rule set the flat-rate service and package directly, which captures a good share of it without anyone touching an order.
The audit has to come first, and it asks two questions of each order rather than one: would the contents have fitted the box, and would the flat rate actually have been cheaper than what you paid. Only the second settles it. An order can qualify for a flat-rate box and still be more expensive in it, which is the part that gets skipped when people hear flat rate is cheap. What you want is a profile where both answers are yes most of the time. If there isn’t one, stop — the rule isn’t worth building, and the audit has answered the question either way.
Then be clear about what the rule is, because it’s blunt in two directions. It over-applies, forcing flat rate onto every order matching the profile including the ones where your standard rate would have won that day — which is why the audit has to show it winning most of the time first, since the arithmetic only works if the orders you get wrong are the minority. And it can’t check fit, because the rules engine holds the order’s total weight rather than the dimensions of the items in it, so some orders will reach a packer with a box the contents don’t go in.
Neither is a reason to skip it. Directionally right on the heavy, far orders recovers most of an amount you’re currently not recovering at all, and the failure is at least visible — a packer holding a box that doesn’t work is a complaint you hear about, unlike the misses above, which are silent. How to reduce shipping costs in ShipStation sets out the audit and the rule step by step, and the FedEx One Rate case works the same way.
Decide packing and rating together. This means a layer that reads the full order, knows the dimensions of your SKUs and your packaging, solves the fit, prices standard and flat-rate options against each other in one pass, and writes the winner back to the order.
That last one is what String does. We price each order against your standard rates and every flat-rate option simultaneously and pick whichever is actually cheaper — because we do cartonization and rating as one decision instead of two. The order shows up in ShipStation with the box, carrier, and service already set, and your team prints the label like always.
If you want to know whether this is worth anything on your order profile, the self assessment works it out from two standard ShipStation exports and never leaves your browser.
Can ShipStation compare USPS Flat Rate against standard rates automatically? Not as a single decision. ShipStation’s documentation states that “Rate Shopper only compares services. It does not compare different package types.” It compares services for a package you’ve already set, and ShipStation’s own documentation requires the package dimensions rule to run before the Rate Shopper rule. Whether a flat-rate box would have been cheaper depends on which box you pack into, which is decided upstream of rating.
Does ShipStation Rate Shopper include FedEx One Rate? Rate Shopper compares the carrier/service combinations you’ve made eligible in your rule. Because flat-rate programs require packing into the carrier’s specific box, eligibility interacts with your package configuration. Check your own account’s eligible service list — this varies by account, region, and plan. The One Rate case in detail, including the stop-gap rule worth building if it’s real volume.
Why does my rate shopper never pick flat rate? Almost always because your automation rules set a package or dimensions before the Rate Shopper rule runs. Once the package is set to your own carton, the carrier’s flat-rate program is off the table by definition, regardless of price.
How much does ShipStation Rate Shopper cost? It’s plan-dependent, and the terms have changed over time — Rate Shopper has been bundled with higher-volume plans and offered as an add-on to others, with custom configurations gated to a higher tier than the three default strategies. Check current pricing with ShipStation directly rather than relying on any third-party figure, including ours.
What’s the difference between rate shopping and cartonization? Rate shopping picks the cheapest service for a given package. Cartonization picks the best package for a given set of items. They’re separate problems and most tools solve them separately — which is fine, until a carrier program makes the price depend on the box, at which point they have to be solved together.
Does ShipStation Rate Shopper work outside the US? It’s available for US, UK, Canada, Australia, and New Zealand accounts. Other countries aren’t supported as of this writing, and some capabilities (like custom package types in Rate Shopper rules) are documented as region-specific within that list.
Knowing how a carrier prices a package is useful. The harder problem is making the right packing and rate decision on every order, when the correct answer changes with the contents, the destination and the rate card.
String builds and maintains that logic inside existing ShipStation accounts, for operations shipping roughly 6,000 orders a month or more. Below that, ShipStation's native automation and accurate product dimensions will get you most of the way there.
Eight patterns that show up when an operation needs custom shipping logic
