I have a 20% off discount that I bought years ago. I can't find how to deactivate the 15% Main shop discount buff anywhere.

There isn't a way to do that, it'll expire in a few hours.

Are you talking about having a discount card?

The buff and having a discount card in your inventory stack.

[edit] Stack might not be the correct word, because you don't get 35% off if you have a 20% and 15%... but you will get a discount from your discount card, and then a discount from the buff.

When I buy something, I only get the discount from the 15% discount buff but not the 20%. This is so aggravating! I basically wasted my money when I bought it. I took a break from Subeta for several years and this is making me not want to play again. Eh.

You SHOULD get more than 15% off, so if you're only getting fifteen it's a glitch. Like Reya said, you get two discounts.

did you double check the discount price? if you have 20% discount card and 15% discount buff, the formula looks like this, final price = original price x 0.8 x 0.85. if it wasn't the case, perhaps file a ticket so it can get sorted out by staff :D

It takes both discounts, but your card one is far less obvious...

I see you have a discount card! I'll give you a discounted price of 334 sP! Mystery Pie has been added to your inventory. Your main shop discount buff gave you 15% (74 sP) off the main price!