Bitstack calculates the sum of all your roundups throughout the week and debits your payment method at the end of the week for the total of roundups of that week. Unfortunately, Bitstack does not work with deferred debit cards because the transactions do not appear on your bank statement during this period. However, please note that Bitstack calculates roundups on all transactions associated with your linked bank accounts (card transactions, direct debits, wires, etc.). Some transactions will therefore be taken into account.
Please note that it is also possible to schedule a recurring buy (weekly, bi-monthly or monthly) by card so that you can still save automatically in Bitcoin.