Terrible, terrible customer service!!!

I bought a $75 worth of dry cleaning for $30 Groupon for this shop. When I I went to pick up my cloth and presented the Groupon, both the clerk and the manager became extremely rude. They told me that I needed to present the Groupon when I dropped off my cloth for the Groupon to work. I told them that such a rule was not stated on the coupon. They pointed to a sign posted on a wall stating that "Groupon must be presented at drop off." I told them that they cannot just post a sign after the purchase of the coupon to change the rules, and the rules must be stated on the coupon. The owner then spent over 1 minute scanning the coupon, and finally pointed to:

"Present coupon at arrival"

under instructions. I told the owner that the point does not clearly state whether I needed to present the coupon at arrival when I dropped off, or at arrival when I picked up. Upon hearing that, the owner practically yelled at me and called me stupid by saying that "all the world could understand that, and only you couldn't!"

So I decided not to bother dealing with the owner. I contacted Groupon, and Groupon's staff agreed with me. Groupon's staff tried to contact the shop owner on my behave to resolve the issue but the shop owner just ignored them. After one week of trying, Groupon's staff finally gave up and issued a refund for me. But now I have to pay $75 to get my cloth back instead of $30, the original agreed up price. I really don't want to pay one extra cent to this shrewd business, but I guess I have to if I want my cloth back...