My default position for these things is that I don’t understand something about them. But from the Fast documentation for sellers [ https://help.fast.co/hc/en-us/articles/360057446093-What-if-I-don-t-use-BigCommerce-WooCommerce-as-my-e-Commerce-platform- ] it seems Fast is only available for BigCommerce and WooCommerce. I am guessing that originally it was BigCommerce’s answer to ‘Shop’ from Shopify.

Pricing for standard sellers is 2.9% + 30c of every purchase, plus an additional 1% for international cards. Add that to Stripe costs and it starts to mount up. For that reason I can see Fast being subsumed into Stripe. Fast does say lower pricing for big sellers, but not what big sellers are.