They're available for a wide number of memberships and affiliations. If you'll be using a coupon or an ad, read the fine print carefully. Sometimes the car advertised is only available on a 'limited basis'. That could mean that by the time you get there the special won't be available. Find out when you make your reservation.
Taxes can vary even within the same destination area.
Airport surcharges are common. The airport charges the rental company and they pass it along to you. That will translate into a cheaper rate if you rent from a site outside the airport. Find out in advance. Don't forget to ask about fees for additional drivers.