> ## Documentation Index
> Fetch the complete documentation index at: https://docs.withflex.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Introduction

> Welcome to Flex! The easiest way to start accepting [HSA](https://www.healthcare.gov/glossary/health-savings-account-hsa/)/[FSA](https://www.healthcare.gov/glossary/flexible-spending-account-fsa/) Payments

Flex offers a hosted checkout page to integrate HSA/FSA payments directly in your existing payments flow. Flex offers two paths to accept HSA/FSA cards at checkout.

## Auto Substantiation

If you are an ecommerce store selling HSA/FSA eligible items, you are required by the IRS to use an [Inventory
Information Approval System (IIAS)](https://www.sig-is.org/resources/faq/programs/iias-certification#WhatisaninventoryinformationapprovalsystemasspecifiedbytheIRS). This system
must auto-substantiate HSA/FSA transactions at the point of sale in order for consumers to use their HSA/FSA cards. Flex
offers this system for our merchants.

## Letter of Medical Necessity

If you are a wellness business (fitness, nutrition, sleep, etc), your product or service may not be explicitly eligible
by the IRS. Consumers must receive a Letter of Medical Necessity from a physician deeming your product medically necessary
before you can accept HSA/FSA funds. Flex is able to issue a Letter of Medical Necessity directly in the checkout flow, providing
a seamless and compliant checkout experience for your users.

## Integration

Flex provides an [API based integration](/developer-guides/integration/checkout/how-checkout-works), a
[Shopify payments app](/developer-guides/integration/shopify/overview), and a
[WooCommerce plugin](/developer-guides/integration/woocommerce/overview), making it easy to get started!

## AI assistants

Connect Claude, Cursor, Windsurf, or any other Model Context Protocol-compatible client to your Flex account using the [Flex MCP](/developer-guides/integration/mcp/overview). Look up payment intents, draft dispute responses, reconcile captures, or manage subscriptions from natural-language prompts.