Understood, and the VISA website confirms that what you are experiencing is designed into the system, although the 20% add-on may not always be automatic. You would need to ask the server to submit the authorization for 17% less than the real amount, but I don't expect they will agree to do this unless you are a regular patron.

FOX85's suggestion should work if you eat at the types of restaurants which sell giftcards at supermarkets or pharmacies.