I did the Skyservice non stop from Toronto last year and, even though it was the cheapest option again this year, I would have gladly paid more to avoid going though US customs at YYZ on the way down and, even worse, whichever 'in transit' airport you travel through on the way home.
Just a couple of points that might be worth noting... the flights are a charter on Thursdays only and you can travel on any Thursday from the middle of December to the beginning of March. There is no limit to the number of weeks in that time frame.
The flight to Belize last year was not non stop. There was a 45 minute stopover in Cancun for Mexico bound passengers to deplane and Toronto bound passengers to board. Those going to BZE did not get off the plane. The return was non stop.
You may be able to get the flight for less than on the Signature website if you go through Holiday Outlet or another of the wholesale sites.