I have a 20 Percent Discount Card in my inventory. But now when i buy things {in shops} i won't get the discount.

Purchased Dark Water Potion

Dark Water Potion I see you have a discount card! I'll give you a discounted price of 272 sP! Dark Water Potion has been added to your inventory.

Your main shop discount buff gave you 15% (69 sP) off the main price!

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It's working for me. Have you read it carefully when you buy something? If you look above you can see there are two separate lines for discount when I purchased my item.

It was a few days it didn't work. I have it already so long i know where i have to look. But a few days it didn't work. Not it is working again.