What time do you arrive in Belize?
There is a Tropic Air flight that departs from PGIA at 3:00 pm...http://www.tropicair.com/reservations/FlightSchedule.cfm
There is an express bus that departs from the water taxi terminal in Belize City at 2:30 pm. That bus takes about 5 hours to get to Flores; US$25.
There are a few transfer services.
Or, you could get there by using a combination of local Belize bus, taxi, Guatemalan bus - might be a bit more than 5 hours.
There are more details about buses on my site:http://belizebus.wordpress.com/