DoorDash Promo scam?

This is so weird. I did a promo that was like if you order this many times you get a 30 dollar coupon. I did it. They sent me a code for 3 10 dollar coupons. I used it once and tried again and it doesn’t work. I call costumer service and they’re telling me if I can’t prove it im screwed (even tho I have a receipt having used it already) I can’t seem to find anything in my app about the promotion