I've never heard of that. That would be called a bait-and-switch, and it's highly illegal. And I can't believe that Alamo's web site would have such a huge hole in it when it works fine to prevent you from using expired codes or coupons, or codes or coupons that don't apply to the class of car you're reserving, or at a particular location.
I will be armed with a printed sheet showing the rate, and have the coupon and my BJs and
Sams club cards in hand, so they better honor what they booked on the web site, or they will hear some noise from me when I get home!
Worst case scenario is that they wouldn't accept both together, and I would pay an extra $20 on my rental.