when I was younger and I had all the time in the world, I used to enter all these sweepstakes and contests that said you could buy X and then be entered into a prize pool/raffle/etc. the pro tip was to read the fine print, Ctrl+F no purchase necessary, and enter using the "alternative method"

my guess is that there must have been some gambling law requiring that there be a method to enter for free.

never got the top prizes but our mailbox was full of random things that confused my parents.