FeeLayer vs. legacy surcharge apps
Most surcharge apps on the App Store were built before Cart Transform Functions existed. They inject JavaScript or hang a Checkout UI Extension off the checkout page. Either way, a customer who taps Shop Pay, Apple Pay or Google Pay goes straight past it. That's the difference the table below is really measuring.
Everything the leaders offer, at a flat $9/mo
These are the surcharge apps you'll actually run into on the App Store. Every check below comes from public listings, vendor pages and user reviews as of April 2026.
Cart Transform Functions
Checkout Validation Functions (2nd enforcement layer)
Bypass-proof on Shop Pay, Apple Pay, Google Pay
US state compliance auto-waive
Theme App Extension (storefront disclosure)
Abandoned-cart safe (no draft orders)
Pricing model
Monthly price (entry plan)
Source: Shopify App Store listings, vendor websites, and user-review analysis, April 2026. Features reflect default behavior on each app's entry-level plan; higher-tier plans may add capabilities that are only listed on paid tiers. Pricing shown is the lowest public plan, excluding free trials.
Most fee apps get bypassed at checkout.
An app built on JavaScript injection or a Checkout UI Extension lives on a page that Shop Pay, Apple Pay and Google Pay walk straight past. The fee never gets added. FeeLayer runs as two WebAssembly functions inside Shopify's own infrastructure, so the charge is applied server-side before anybody pays.
JS injection + UI extensions
What most Shopify surcharge apps are still running today.
- Shop Pay, Apple Pay and Google Pay skip the UI extension entirely
- A customer can delete the fee line out of the cart
- A bundle app gets installed and the Cart Transforms collide
- Behaves one way on Android and another on iOS Safari
Cart Transform + Validation Functions
Built on Shopify's current checkout architecture from day one, not retrofitted onto an older app.
- The total is recalculated server-side every time the cart changes
- Checkout Validation throws out any order whose totals were tampered with
- Both functions run before payment auth, so express checkout is covered
- A Theme App Extension puts the disclosure line on your storefront
What to evaluate
Four questions worth asking about any surcharge app you're looking at, ours included.
- Does the fee survive express checkout? Shop Pay, Apple Pay and Google Pay skip most of the places a legacy app injects its fee. Applying the fee server-side with Cart Transform Functions is what makes it land every time.
- Is there a second enforcement layer? Checkout Validation Functions throw out orders whose totals don't add up. Without one, a determined customer can strip the fee off before paying and the order still goes through.
- What happens on an order shipping to Connecticut? Surcharging is banned there, and in Massachusetts and Maine. Card-network rules also cap what you can add per transaction everywhere else. You want the app to drop the fee in those three states on its own and hold every other order under the ceiling.
- Flat or tiered? A tiered app charges you more as you sell more, which is a strange thing to sign up for. Flat pricing costs the same in your best month as in your worst.
Try FeeLayer on your store
Request early access and run it next to whatever you're using now.