We have used Avis and National for all our (MCO)Orlando trips ( Both in Airport and a BREEZE getting paperwork and car ) Check ALL the clubs and membership cards you have for any rental coupons or codes for car rentals. Also, put in every combination you can with them (Computers on the web site are not human and do make "collective" mistakes at times ( WINK WINK ) Remember, have the coupon or card(s) with you when you arrive, some people will look the other way and some wont. Last, If you do use the internet - remember you can always cancel your reservation and grab a better deal right up to the last minute ( with no penalty), so never stop checking for deals in the papers or at their web site.
Cant speak for the other on-site rental companies.
Good Luck!
8')