So the reason you hear about "sweepstakes" on the radio and shit is because the Federal government is a criminal cartel that make sure that they're the only ones allowed to run lotteries, which are prize + chance + consideration.
So if you allow people to enter without purchase (mail-in), then you've removed consideration and it's now prize + chance, which is legally not a lottery.
"No purchase necessary"
Government is legalized broad daylight mafia.