Use a summary field in the table (credit card payments) defined as Sum (Credit card payments::Amount [or whichever field defines the amount for any individual payment]).
You can perform a find (omit) for the 'checked' value in your 'check' field. Your Sum field (if you've placed it on a layout) should now sum the amounts for the found set (all unchecked).
A layout designed for list view may suit your needs. If you include a sub summary part you can place your summary field in it. Remember you will have to sort the records to see your sub summary part and it will have to be defined as being sorted by one of the fields you sort the records by.
You may need to automate the 'check' process on the click of your button - see Set Field and Loop script steps.
Hope that is helpful,
Thanks for your help. It's really appreciated. I have added a summary field. The problem I run into now is how to search for unchecked records. If I leave the radio button unchecked Filemaker tells me to enter valid search criteria. I can search on checked records obviously, but how to search for unchecked records when having to leave all remaining fields empty as well because the unchecked field is the only search criteria I need.
Edit: I found it. I can search for checked records and leave them out of the search resulting in all unchecked records.