What's the problem? The problem is you're not merely reselling the decal, you're selling a new product -- and you're using the logo/decal to distinguish it from similar products (by associating it with a sports team). Why else include the logo? Custom-making the logos so they looked slightly different wouldn't help -- you're still relying on team associations -- and probably would make it worse since your alterations would now be diluting the mark. (By the way, we answered a related inquiry from a crafts artist, here.)
Toe-stepping Dept. When companies pay big money to become cornhole board licensees, they expect some degree of exclusivity and they expect the leagues to police unauthorized uses. That's why you got that letter. Stating that you're not affiliated with the sports franchises (known as a disclaimer), is not likely to have much impact on your case. Sometimes prominent disclaimers -- for example, large lettering affixed right near a logo -- may mitigate a problem, but it's unlikely to succeed here. (we talk about disclaimers more, here.) The leagues have too much at stake and will vigorously pursue all potential infringers. So, unless you're ready to take them on, the Dear Rich Staff suggests that you either give it up or move your enterprise off the grid.