Shopify Subscriptions - Flex Documentation

Overview

Shopify does not allow third-party payment providers like Flex to directly accept payment for subscriptions. Flex uses a post-purchase reimbursement flow so customers can still use their HSA/FSA funds. With this setup:

• Customers complete subscription purchases with a standard payment method (credit card, ShopPay, or any Shopify-supported method).
• On the Thank You page, the Flex reimbursement block invites them to get reimbursed with their pre-tax HSA/FSA dollars.
• They complete a quick consultation with Flex to verify eligibility, then one-click submit a claim from the Flex Portal.
• Merchants are billed per reimbursement letter generated.

Customer Journey

Flex Shopify Reimbursement Flow - YouTube

1. Customer completes subscription checkout using a standard payment method.
2. On the Thank You page, they see the Flex reimbursement block with a call-to-action to get reimbursed using your pre-tax HSA/FSA dollars.
3. Clicking Get reimbursed opens a Flex consultation to verify their eligibility.
4. Once approved, the customer can one-click submit a claim directly from the Flex Portal and receives their Letter of Medical Necessity (LMN).
5. The customer submits the itemized receipt alongside their LMN to reimburse each invoice.
6. The Letter of Medical Necessity is valid for 1 year.

Integration Steps

Make sure to reinstall the Flex Marketing App to take advantage of the subscription solution.

1. In Shopify Admin, go to Online Store → Themes and click Customize.
2. From the top dropdown, select Checkout and customer accounts, then switch from Checkout to the Thank you page.
3. Click Add block, select the Flex reimbursement block, and drag it where you’d like it to appear (e.g., just under the Order status).
4. Click Save, then exit the editor.

💡 Tip: The block has an optional Payment link field. Leave it blank — it’s only used by merchants on the legacy Flex Marketing App. If you recently installed or reinstalled the app, you’re on the latest version and don’t need it.

Limitations

• Shopify subscriptions cannot use HSA/FSA cards directly — customers pay with a standard method and get reimbursed afterward.
• Customers must complete a separate reimbursement step after each invoice.
• LMNs may only be used to reimburse invoices dated the same day or up to one year in the future.

After that, customers should request a new LMN.

Example Email & LMN

Customers receive the LMN after completing the reimbursement consultation and will also receive an itemized receipt from Flex. They can submit these documents to their HSA/FSA administrator for reimbursement.

FAQs

What is a Letter of Medical Necessity (LMN)? A document signed by a medical professional verifying that a product/service is medically necessary for HSA/FSA reimbursement.

Can customers use their HSA/FSA card directly for subscriptions? No — Shopify does not support third-party payment methods for subscriptions. Customers pay with a standard method and use the reimbursement process afterward.

Customer’s claim was denied — what should I do? Ask the customer to confirm eligibility with their HSA/FSA provider. If denied, have them share the provider’s written response so we can review. (FSA rules can vary by employer.)

Can customers buy now and get reimbursed next year? Usually no. Most FSA providers require purchases in the same calendar year. Some allow a short grace period—customers should confirm with their provider.