If I have both a 20 and a 30 percent card in my inventory...will both work to take off sP if I buy from a main shop? Or just one of the cards?

No the cards are not additive. I think the higher discount gets used, but it may be the first card that gets in your inventory. Either way, only 1 card will be used.

The regular 15% buff that comes up every once in a while when you shop is additive with either card though.

Thank you so much for your help on that!