It seems to me that, given the bespoke nature of so much stuff on Etsy, they should absolutely have a pay-up-front option for items that are being manufactured to order. In any other kind of bespoke trade, putting down a large deposit or even full prepayment would not be considered unusual at all.

Sure, there would be risks associated with doing this as a buyer, but I bet a lot of people would do it for a reputable and trusted vendor. Especially if the alternative is seeing large additional fees added to the order.