Butterfinger offer required some 3rd-party to look at the receipt to either accept or reject. Promotional companies do this all the time for rebates and the like, and that's why they hired to do that part.

Mogstation has traffic like anything else in the game, people are buying things so often. Credit card payments need to post, delivering mail to characters is a server hit, plus account wides, and especially when it's new and hot.