Squeak:
That’s a real doozey!!
Regular credit card interest is not like the ordinary Ribis, because the interest involved is only conditional. Only if the funds are not returned by a certain date, is a surcharge imposed. This payment is more like a fine than Ribis. For you to arbitrarily “fine” your relative in order to teach a “lesson”, you are creating an established interest fee.So, unless you have a really detailed and codified Heter Iska, you’re a usurer.
Another factor to consider is this: In general anything ostentatious, even if the arrangement had existed prior to the loan, may not be performed by the borrower for the lender. Your relative can’t buy you a fancy Shalach Manos basket to you, because it can halachically be considered interest. You can’t borrow his car either, because it can be considered like he is forced to treat you this way due to the loan.
Ribbis is ribbis regardless whether or not you were joking around or teaching a lesson. There are other ways of teaching responsibility that don’t violate ribbis laws. Interest is not something to play around with.