The problem is Automatic payment. it's monthly payment that Etsy takes automatically based on estimated amount of sales, not actual amount. Usually it's rounded to a larger amount, the difference rolls onto the next month. It's favorable to Etsy, but as a seller I prefer to have actual monthly billing. If you can you should change billing settings and opt out of automatic billing (all new sellers are opted in by default, because Etsy tried to avoid "hit and run" sellers and scammers who would take customers money and will close account without paying fees. I believe sellers can opt out after few sales unless Etsy changed it for good).
Or even more of a scary thought... if the fee is an estimation of how much I might sell, they wouldn't charge me $100 next month even if I don't sell anything right?
I just read through all of their terms and selling fees section and they only mention a 3.5% fee, and nothing about this automatic payment. I tried to update my settings but there is nothing regarding opting out of an automatic payment. I am on the selling app though so maybe I can find it on the desktop!