I stock up on Apple gift cards when they're 20% off at Costco. I'll buy $200-$300 and add it all into my account. So I'm getting 20% off all purchases, effect

I stock up on Apple gift cards when they're 20% off at Costco. I'll buy $200-$300 and add it all into my account. So I'm getting 20% off all purchases, effectively, until the credit runs out.