I "paid" during the Sam / Matt shift change at 4 PM. Sam took down my debit card info. Matt and Sam figured out the Megabattle registration and forum sign-ups. The money transaction was supposed to be called into Abington at some point, and it probably was.
I also bought a Lizardmen Stegadon and was allowed to walk out of the store with it (and the receipt), so everything appeared to be everything.
If there ends up being any sort of problem, I'd reluctantly accept a new Arachnarok for any inconvenience I may suffer.