Scam implies you were duped into believing you were being sold a product or service, when in fact you were not sold what you were being sold. So in this case it would be if you were rented a car and charged for it, but no car was actually rented to you.

A ripoff is being overcharged for a product or service, and this is somewhat subjective. What you may believe to be a ripoff may be perfectly satisfactory to someone else.