Starbucks Centrale

Well I brought my tumbler and I took Ice Americano venti size. The original price is 2,80€ and it supposed to be discounted 20% off because of using my tumbler. The thing is the staff(Giuly) did 0.2€ off and I paid without noticing the price mistake. Then I wondered if she calculated wrongly or it’s me who misunderstood the percentage of discounts. So I asked is it 10% off or 20%? Then she said 20%. I complained that it shouldn’t be just 20 cents off if u do 20% off from 2,80€. Then she said it’s a mathematical thing and treated me as if i was stupid or sth. I showed her how the price should be, which is 2,24€. But she didn’t listen to me at all and keep telling me “matematica”! Wow 😯 I was so shocked and felt terribly bad. It’s not because I payed more than that, but I was treated like a FOOL. As a Starbucks lover, It’s really a shame that they don’t offer good service to customers.