Fast made what was manifestly the wrong decision, both in hindsight and in foresight (I'm happy to post my WhatsApp screencaps from a year and a half ago when I was exploring starting a co in that space). It's not possible to build a hands-off 'platform' like Stripe in that space - integrations are complex, and so integrating a large number of small clients predictably bogged them down till they ran out of money.
(That's not the only reason, I'm sure. It was also a shitty implementation of one-click checkout: most saliently, it wasn't one click. Anyone who actually tried Fast at the time - and I & my prospective cofounder shared the few tiny client websites either of us found like they were rare gems - could see that it was a terrible implementation, in a space where UX is literally the entire game. Many actually did, e.g. this guy from back then: https://chrisfrantz.com/checking-in-on-fast/)