Unexpected charges, with no real explanation as to why

Arrival wasn't great as from LHR T3, the details on whom to call from bus stop 21 are so small that you can easily miss them. Proper signage would go a long way. I waited around 15 minutes to be served, as the staff were very busy. When it came to payment, they lumped an unexpected 84 GBP payment on my car hire because my return ticket did not correspond to the date of car return. I rented the car for two days to visit family and then am continuing to work in the UK for a few months. The woman behind the counter said she would waive charges if I provided an original proof of address from within the UK in the last 3 months - yeah right! She would not accept a photograph or reverse the charges if I provided an original copy upon return. Really terrible and disappointing customer service. All in all, I was looking forward to my Goldcar experience but the "plus, plus" mentality where they sting you with baseless charges will mean that I never use them again.