This can get confusing quickly! And first, a disclaimer, we're not lawyers. If you're wondering about what campaign to run and want to make sure you're in compliance with local regulations, we definitely recommend you speak with a local lawyer.
BetterWorld provides a tool called Giveaways. That is legally a charitable sweepstake where people can donate and be rewarded with entries. They can also enter for free or, in some cases, 'share-to-enter.' (Share to enter is available with Sliding Scale style Giveaways) A key distinction is that, with all our giveaways, people don't technically have to make a purchase to win. (If you've ever come across a 'no purchase necessary to enter' clause, this is the reason.) People can find this free entry option by looking at the official rules / additional rules & guidelines at the bottom of the page.
To note, we find that 98% of users will make a donation for a chance to win. Donors want to support your organization!
We wish we could advise on this more, but the law varies from state to state and gets rather complicated, rather quickly. Any further questions about your specific campaign would be best answered by a qualified attorney or legal advisor.