High charge for wrong dimensions

I paid £25 for a parcel delivery and accidentally entered a dimension that was too small.. If i had entered the correct dimension, then i would have had to pay £32, so a difference of £7 approx... Because ParcelHero/UPS audit all the sizes/weight, they detected that my parcel dimensions were not as stated when i purchased parcel delivery.. I am now being charged an additional £19-30 on top of what i paid originally, which means ive paid £44.50 for an item that should have cost approx £32. I feel like i have been totally ripped off and worse, i feel that ParcelHero dont care less. The appeal i tried to make was ignored and fobbed off because i was unable to prove that there was an error in UPS audit (Like as though anybody could ever prove that!!!) Anyway, its sad really because they have lost a customer, i will just use somebody else for my fledgling ebay/amazon business.. They will lose a lot more than what they charged me for.