Between Amsterdam and Paris? What customs?
If you are driving by yourself, I'd consider taking Thalys (
http://www.thalys.com ). The cost of renting a car specifically for this journey plus the cost of petrol (about 1€ a litre, if I recall correctly) and the cost of French tolls would easily surpass a first class unrestricted one-way ticket on Thalys (€137). Plus it only takes about 4h 10m by train from Amsterdam to Paris, about half an hour faster than driving.
If you do drive, I agree that it's a nice way to see the small towns. Don't miss Antwerp!