Companies big enough to give coupons should have attorneys to make certain someone couldn't take advantage of poor wording on the coupon to cause a financial problem for the issuer. That said, your example certainly is confusing, and only knowledge of the particular product would help make clear what could be meant by 'transaction' and 'trip'.
My guess would go something like this: The coupon is for a grocery item- let's say a can of soup. The issuer wants you to try their new soup, so they issue a coupon good for one free can of soup.
What the issuer seeks to avoid would be:
A. Someone printing up 10 coupons and bringing 10 cans to checkout, expecting to get all 10 of them at no cost.
B. Denied the above scenario, the shopper gets one can free, walks out the 'Out' door, back in the 'In' door and gets another single can of soup. Repeat 8 more times.
Student or Learner