Considering that the discount buff for the main shops is 15% and the discount card I have is more than that, is there a way to disable the buff or force the main shops to recognize my discount card? As it stands, it always uses the discount buff, which can be pretty annoying.

It actually uses both discounts.

This is the message I got after buying something with the buff + discount card:

Only the buff's text is highlighted, but both discounts are mentioned and if you do the math, both do come out (I want to say the buff comes off first but I'm not sure that's accurate)

I don't think it matters which comes off first, it'll be the same amount either way.

I didn't even do the math to check, I just saw the 15% and stopped RSing. Thank you both so much!