Options only charged for after

On May 20th I booked a avis car for the period 15 - 17 August. I selected the option for a child seat while making the booking. I selected the prepaid option and was given a price. I paid. I took the car and returned it on those dates. I then got an additional charge on my account. When I queried it they indicated that it was for a small amount of fuel and the child seat. I argued that it should have been included in the original amount as I had selected the item as part of the quote. They indicated that it was an over the counter product and was billed separately. That may be the case, however there was no mention of this anywhere, so how am I supposed to know this? I am now over budget for this. I really feel that I should get a refund for the amount....