OT - Kinda.
Is it possible to do the trade in thing after the order has been placed?
I do not think so but maybe contact Google and ask.. I assume the only way would be cancel existing order / return and then re order with the trade in...
Seems to be Google is being cheap with it's trade in's, but I guess Apple is the same way. $350 for and iPhone 8 in great condition. Can get more for that and Pixel on Swappa.
At least the Google trade-in is guaranteed.
Selling it yourself, there's nothing to prevent someone from damaging your phone and returning it for a refund. I actually had that happen, so never again. I'll take the guaranteed amount.