Burst vs Flex: HSA/FSA for Your Business, Compared
Burst vs Flex for merchants: a 30-minute self-serve setup vs a checkout integration. Compare pricing, recurring billing, and who can offer each.
6 minute read
TLDR;
Flex: a checkout integration on their payment rails, on terms you learn through their sales process. Built for online merchants.
Burst: self-serve, live in 30 minutes, free for merchants with no contract and no revenue share. Works for online, in-person, and membership businesses. Your customers pay you the way they always have. Burst handles the letters and the reimbursement, and refunds the one-time letter fee to the member if a letter isn't issued or a claim is denied.
(Shopping as a consumer, not a merchant? You can get a letter for any purchase directly from Burst, or see how Burst compares to Truemed and Flex for shoppers.)
Burst vs Flex for merchants
Burst | Flex | |
|---|---|---|
What it is | LMN and reimbursement platform, no checkout involvement | HSA/FSA card payment infrastructure in your checkout |
Getting live | Self-serve, about 30 minutes | Checkout integration on their rails, typically weeks |
Engineering required | None; checkout, POS, and billing untouched | Yes; their checkout in your stack |
Merchant pricing | Free for merchants, no revenue share | Not public; learned through their sales process |
Built for | Every business: retail, gyms, spas, studios, online or in person | Online merchants running their checkout |
Recurring billing | Your existing billing, untouched; no card rails | Limited support |
Member cost | One-time letter fee, paid by the member | $15 LMN, per their published programs |
Customer protection | Automatic refund if no letter is issued or the claim is denied | None advertised |
What Flex is for a merchant
Flex is a payments company (Y Combinator S23) whose checkout lets your customers pay with HSA/FSA cards, substantiated in real time, split carts handled, subscriptions re-validated at every renewal per their documentation. Equinox is the flagship partner, which tells you who the model courts. It's real infrastructure, and it asks what infrastructure asks: an integration project, their rails under your revenue, and commercial terms that aren't published anywhere you can read before entering the pipeline.
What Burst is for a merchant
Burst adds the HSA/FSA benefit without becoming part of your payment stack. Signup is self-serve and takes about 30 minutes; nothing changes in how customers pay you. Burst runs the Letter of Medical Necessity side with licensed clinicians (letters within 24 hours of approval), and Burst files the reimbursement claim for the member; their plan administrator pays the reimbursement out of their FSA, HSA, or HRA to them.
Burst is free for merchants. Members pay a one-time letter fee. And the offer carries a guarantee Flex doesn't advertise: if a clinician doesn't approve a letter or a claim is denied, the fee refunds automatically, so recommending Burst never costs your customer anything.
The differences that matter
Integration project vs 30 minutes
Offering Flex is an engineering decision with a sales process in front of it. Offering Burst is an account signup. If you have a developer, a roadmap slot, and patience, the first is possible; if you want the benefit live today, Burst is the one that needs no engineering.
Who can even use it
Flex presumes an online checkout to integrate into. A front-desk gym, a med spa, a studio on custom billing, a farmers-market retailer: outside the model entirely. Burst assumes nothing about how you get paid, so every business type onboards the same way.
Whose rails your revenue runs on
With Flex, HSA/FSA purchases and renewals flow across their card rails with real-time substantiation, because the card networks require that check on every HSA and FSA card charge. With Burst there are no card rails in the loop at all: members pay through your existing billing, and the letter plus their plan's rules covers the expense. Nothing new in your payment path means nothing new that can fail in it.
Terms you can read
Burst is free for merchants. Flex's merchant terms come through their sales process. Burst is free for merchants.
When Flex makes sense
You're an online merchant that specifically wants HSA/FSA cards accepted inside the checkout itself, at a scale that justifies an integration on their rails, and you have weeks to build the integration.
When Burst makes sense
Every use case: in-person, online, and hybrid businesses, membership billing, owners who want it live this week, and anyone who wants to know the price before the pitch.See how it works or go live now.
Frequently asked questions
What does Flex cost a merchant compared to Burst?
Flex doesn't publish merchant pricing; terms come through their sales process (typically a rev share %). Burst is free for merchants, with no contract or revenue share. The one-time letter fee is paid by the member.
Do I need a developer to offer Burst?
No. There's no technical integration; your checkout, POS, and billing stay exactly as they are. Partners are live in 30 minutes.
Can a brick-and-mortar business offer Flex?
Flex is built around online checkout integration. If your revenue happens in person or through your own billing system, you're outside their model. Burst works the same for online, in-person, and hybrid businesses.
How does recurring membership billing work with each?
On Flex's rails, each renewal is re-validated in real time per their documentation, because IRS rules require substantiation of HSA/FSA card charges. With Burst, members keep paying dues through your existing billing on their normal card, nothing changes in your process at all, Burst handles everything else.
Can members get letters for past purchases?
No. IRS rules require the letter to be dated on or before the purchase. That binds Burst, Flex, and every other provider equally.
Related guides
Offer HSA/FSA savings this week
Self-serve setup in about 30 minutes. Free for merchants, no contract, no revenue share, no integration.Get started with Burst or see how it works.
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