I made this very mistake T_T I am also very disappointed and SOL to get a refund and re-purchase from their website.
While I'm glad serif is one of the few One-price software makers around, I wish it was more apparent that you wouldn't get a product key or exe file to download when you get their software off the Microsoft store.
I have bought other games and other software off the M-store before but standard practice seems to be to provide a product key regardless, so I feel let down about this...
I've had to wipe or replace my laptop about 2-3 times in the past when things went horribly wrong, just having a personal backup of the .exe & your product keys just makes getting back up to speed that much faster