associate different credit cards with every purchase
I think it's fine to do your first idea with adding the id of customer_cards as a field to the Payment_entries table. This then makes it a linking table between Order and customer_cards so you can answer the question &quot;which customer cards have been used to place an order?&quot;.
For scalability, I would advise calling the new column something like payment_method_id in Payment_entries that way it's generically re-usable for other payment_types you may add as you scale.