can anybody point me toward documentation on how to add a credit card (to a customer) via suitescript? Does it perform similarly to the address subrecord?

its a normal sublist, so should actually be easier than an address subrecord

Cool. Maybe I just did something stupid in my syntax that made the error message pop up. You have attempted an invalid sublist or line item operation. You are either trying to access a field on a non-existent line or you are trying to add or remove lines from a static sublist.