An Article Processing Fee is usually charged after acceptance, but you can prepay (deposit) money in advance, which is particularly useful for expiring funding (money that needs to be spent before a grant expires/ends). (Another option in this situation of an expiring grant would be to purchase an institutional membership).
JMIR Prepaid Credits act like real money. Each prepaid credit is valued at $1USD and is redeemable for anything you might want to purchase from JMIR Publications. Each credit you purchase will be valid for 365 days.