No personal experience but I would think a small piece of carry-on luggage clearly purchased out/about on Disney property would be just fine. If any doubt just leave the tags/stickers/etc on.
If for some reason they give you a hard time, just plan to rideshare/taxi as a backup just in case, if an option for you.
I would have them put it in a large shopping bag to avoid any issues. Or since you know you are buying take along a large shopping bag. I know for awhile they were selling reusable bags that would fit that.
Are they shipping packages back to the resort yet? If so I would do that.
Good call on the boat. Especially if mid-day, the boats are rarely crowded at that time. And I think the boat would have more room to accommodate the suitcase. If the boat captain asks, tell them you purchased the luggage at Disney Springs. You should be fine on a boat.