As long as the booking has no cancellation fees, which generally postpaid rates do not, then you do not have to take it

If they are giving you a car which is the category that you reserved, then I do not see what the issue is though. If it is a lower category , then I would be wanting the correct category or a reduction in rate if they are out of the correct category

Even on a prepaid rate, if they are unable to meet the reserved category, I would expect to be able to cancel given that they are not providing what was paid for