No man, now that you've kept the card it has to act as some sort of payment. You've accepted part of the bargain once you decided to keep it. It's like how legally if you said "I'm sorry" you're assuming fault. Trust me on my legal knowledge bro, I grew up in New York and everyone sues everyone up there. Some lady threatened to sue me because I coughed too loud in the library. I'm deaf. Did she think I was aware of exactly how many decibels my coughing was?
Anyway so now that you, a third party of hopefully a legal age for me to send cake batter kisses to, has accepted that and kept it in your possession, I can legally offer you less (because my candy smooches are worth at least 60% of a hershey's kiss, so I only have to make up the 40%)
I'm gonna be the house that gives out apples to trick or treaters.
Get ready for travel tubes of toothpaste, smarties, and fun-sized pretzel packets.