We have an Avis booking that was made through BA and paid in full. When collecting the car there was a balance due - around $60 - that the Avis clerk could not fully explain and suggested could be due to currency fluctuations since when I booked it to now, as I paid in GBP. I should have challenged further but we were in a hurry to collect so I will take it up with BA holidays when I return home. Be interesting to see what they say as from reading this it seems a common thing to add on unspecified vague charges...