I built the first online payment system for my university' frosh week in the early 2000's (before that they only accepted cash). This is pre-shopify/etc, and if you were doing any moderate volume it was worthwhile to use a gateway that went direct to your own merchant account.

They started accepting signups a couple weeks before school started, and on the second day it was running, after several hundred signups (of ~$200 each iirc), apparently some security people from the bank showed up at the orientation office at the school basically just to confirm it was legit. (I don't remember if they suspended the account first or not)

I had built a few online store sites using merchant accounts by then, but nothing that went from zero to that volume so quickly; it was fascinating to see that check in action.

I wonder what volume it would take to trigger such scrutiny today, and what it would look like..