Yes, you can use whatever card you already use for gas, and pair your purchase with other promotions. It’s important to note, however, that if you use a rewards card or other promotion, your Upside cash back per gallon will be reduced to account for the cost of the other promotion.
This adjustment to account for other reward programs is something we've implemented at the request of the gas stations we're working with. They've raised the issue that the Upside program, when layered with their existing rewards programs, can result in them losing money. We do this adjustment as a compromise so that the Upside program works for the gas station, so we can keep them in our app and keep getting you cash back. We figured it was better to do this than to have gas stations leave Upside, leaving you with less choice. And you can still stack programs, getting you more savings on gas than just using one program.