Here's the advanced car booking page at SW: https://www.southwest.com/car-rental...r-rentals.html

In the Promo/Discount Code section, you choose "Add More" to add both an AWD # and a Coupon #. This is where I explicitly left out the Avis Wizard #, in order to (attempt to) ensure that Avis didn't do the very thing that they did.

In any case, you're point is well taken - clearly a significant portion of the blame falls on Southwest - they are the ones who posted a promo that didn't work as advertised. Had I started at Avis, it probably would have worked.

Thanks again!