Hi! Jerry here, from 3204 Auctions. We do ask for customers to buy credits in advance because too many of them were changing their mind after a comic was already bought and submitted for them. The 3-credit minimum is so that I don't have to submit endless invoices every single day (lol). It also allows customers to make last minute requests without having to rush to buy credits at an inconvenient time. You can just have a bank of credits to draw from that never expire! It really works out great for both parties. If a comic fails to get a 9.8 grade, that credit remains in your account, unused.
If we missed a few issues for you I apologize. It was not from a lack of trying. The only time we really miss a request is if the entire batch of an issue is damaged by the manufacturer or the variant you requested is too scarce for us to locate a 9.8-worthy copy.