Actually if you need to check bags, unless you are flying SW or one of the airlines that assigns seating or boarding during check-in, there's really no need to check-in prior to getting to the airport. When you check your bags you'll be issued boarding passes.
I don't know if Sun Country assigns seats at check-in, I think they are assigned when you purchase the tickets (I could be wrong on that) and if that's the case there's really no need to wait in a line to get checked in at the resort only to wait in another line to check bags at the airport.
Anne