Quick follow-up update after my stupidity last week.
When I saw that the PW was on sale this week (of course! just after I bought it!) I contacted amazon CS. I asked them to help by putting the new unopened kindle on my/our account, and also asked them to remove the ads for free as a courtesy, and really to save me returning this one, then reordering another one at the cheaper price. Which would be further complicated as I had used a trade-in of 20% which would not reappear in my cart until amazon received the return back.
So, they did this for me, and super quickly and efficiently. Really wonderful customer service.
And now I can load some books on it for him, and he won't have to look at stupid ads.
And I don't feel cheated for buying when it was full price.