It can be tricky to customize a checkout in PowerStore but if you have the tools it is possible to do. There are probably a couple of ways that you can do this, you can remove the validations on the current form when the particular radio option is selected but this will require some custom coding.
Another way you could do this would be to have a different checkout form when the user chooses the credit option. You could have an new and different form present on the page that will not trigger the server side validations. Once you craft the form on the page in DW you can use client side validations from the CSS Form Builder to add a show/hide layer client validation response based on the selection of the radio button. This would then make it so you have the two different checkout forms and only one or the other would show at a time based on the selection.