My mother uses my credit card to help me build up reward points. She repays whatever she spends to my account. Sometimes she adds a little extra and then she tells me I don’t have to return and I should treat it as a present. Is this a problem of ribbis?
Yes, giving additional money in connection to a loan, even declared as a present, is a Rabbinic prohibition of Ribbis.
Shlchan Aruch Y:D 160:4, Shach ibid., see Bris Yehuda 5:2 and footnote there.