For many candy fundraising is just a natural. Here's what some of the biggest leagues do:
Well before their actual registration dates they let potential players know what the cost to register will be. They also let them know that there is a required baseball fundraiser they need to participate in. They explain that everyone is required to sell candy fundraising items. The amount of candy that needs to be sold is determined based on the actual needs of the league.
Many leagues require each player to sell one or two carriers of candy assuming they will make between $20 - $25 profit per carrier. So if the league needs require $50 in addition to the actual registration fees they require each player to purchase two carriers of candy.
The player then has the choice of selling the candy to make their money back or simply stocking the candy in their pantry and having lots of snacks on hand for the next year.
Of course most of the leagues also allow players to opt out of the sports fundraising campaign by paying the league just the profit that they would have made on the candy or in essence a higher registration fee.
Candy fundraisers work well because the league can simply purchase the number of cases it knows it needs since players need to opt out before registration and then have each player pick up their candy at registration.
There are other leagues that do the same basic required participation in a discount card fundraiser.
So if participation in your league fundraiser is less than you would like consider making it a requirement during registration.