I have no idea. In my case, I have to add the request after the rental. My rezzies are made by Amex via ITN, and I can't change the rez on the avis website because it was made by a TA. (Not even to add an FF program.) But it does claim that you can add them after the fact on the history page and you're supposed to get credit.

Maybe I should try crediting them to SPG next time, because I tend not to follow up on this stuff, but I always notice that there's no entries for the credits in my GP account.

There just aren't that many ways to earn extra GP points.

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OK, I had 8 rentals in my rental history that didn't have a program assigned. I added GP to 4 of them and SPG to the other 4.

When I add them, it accepts them and it takes you to a screen saying that it takes 48 hours for them to issue the credits.

-David