I had a issue with Avis this week, after getting back from a holiday in South Africa.
Booked 3 cars (Cape Town, Bloemfontein then Cape Town again) via BA web site. Paid for all three via Avios points.
I was told about small charges due to fluctuation in exchange rates. Was charged approx. £1 for first booking, £6 for second, which was OK. My third booking I was charged £218!
Upon calling Avis they said that "the BA voucher hadn't been applied" and said that this can be a problem at some locations. I pointed out that the voucher had been applied first time round in Cape Town so why not the second? They had no answer for that.
Apparently I should get a CC refund within 5-7 days. We'll see...