I'm the solo founder of CheapCarRent, an Estonia-based car rental marketplace. Most of the work is not the website, it is connecting rental companies' systems so their cars and prices show up in one search. Here are the places where those systems disagree the most. Maybe it saves someone an afternoon.

1. Locations are the hardest part. One system gives you an airport code, another a free-text branch name, another coordinates, and some give only a city. If you resolve "Vienna" to the wrong country code once, customers will see cars in the wrong city. We now verify that every offer has a pick-up point that can actually be placed on a map, and drop offers that can't.

2. "Deposit" and "insurance" get mixed up. Some APIs return the deposit as an insurance line with a price, and a naive mapper then shows the customer a "paid insurance" item that is really a refundable hold. Always check whether a money field is a charge or a hold.

3. Insurance is not one field.…