I am not 100% certain, but I'm pretty sure in the last 1-2-FREE promo (actually, I think it was the book 3 trips get 1 2-day weekend free), I had issues because my first reservation was reserved before I registered for the promo (i.e., reserved, even though the actual rental was during the relevant promo time period). Eventually, it was a moot point bc i ended up having 4 rentals during the period, so I got my free weekend, but it might be worth triple-checking on this point.