If I go to the supermarket for my parents, spending $100 of my own money, making me a lender. Can my father pay me back $300, as the money is something he tries to help me out with anyways or is it like ribis?
Paying back $300 for a $100 loan forbidden, even if he helps you out anyway. At the time of repaying the loan, only $100 should be paid.
The way to help you out is to first pay back the $100 debt, and later to give you the extra $200 dollars. This is permitted if he helps you out anyway.
Sources: Shulchan Aruch, Yoreh De’ah 160:4, 6; Shach 9 (and other commentaries).