The transaction amounts on your linked cards are used to calculate your Round Ups. If you make a $2.50 charge on your linked card, $0.50 is added to your Round Up progress bar. Purchasing from linked cards is not possible.
Your funding source is the bank account Crumbs uses to make purchases and deposit your withdrawals. When your Round Up progress from your linked cards exceeds $5, Crumbs makes a purchase for you using your linked bank account.