The Pixel 3a and 3a XL just went live on Amazon a few hours ago, but a pretty enticing promotion is already being offered. Buyers of the Just Black Pixel 3a XL can get a free $100 Amazon gift card, which is sure to be much more useful than the $100 in Google Store credit you'll get from Google itself.
The deal is only valid on the Just Black color of the larger 3a XL model, but there's a good chance that's the configuration you were looking for anyway. It should be noted that Amazon is saying the phone "usually ships within 1 to 3 weeks," so if you need the phone soon, you might want to buy one elsewhere. But if you really want the Amazon credit, you might want to click that buy button soon given that it's already backordered, and also be prepared to wait.
It turns out that Amazon is actually discounting all configurations of the Pixel 3a and 3a XL — it just doesn't know how to organize and structure its links. Droid Life found all of the links, seen below: