I had a similar experience with AVIS at Almeria airport in Spain. We landed, were told they didn't have a car in our category (quite common, we accepted that) and we had been upgraded. I rejected the Seat estate they gave us, so they substituted it with a Renault Kadjar, which was acceptable to me. I asked how much the full insurance was for the whole trip, and was encouraged to come round the counter and look at the figure on her screen of - an extraordinarily low - 42 euros for a long weekend (it was about 4-5 nights). Clearly I accepted and the documentation was emailed to me. When we dropped the car off (full of fuel) we were hurried into a minivan from the compound to the main terminal. I later checked my CC statement and we had been charged 260 euros. I then asked AVIS via social media to address this and eventually they admitted their error and a refund came through. Being an AVIS preferred customer and having used them countless times I was less than impressed.