Interchange shopping cart cannot process credit cards. If you have it, you still need to get trusted merchant account from companies that provide credit card processing services. However, You can integrate the credit card processing with Interchange shopping cart.
Some recommended merchant account providers are: revecom.com and 2checkout.com
These companies will take care of all your billing needs.
they take your customers credit cards and send you the funds by cheque or wire transfer to your bank account.