I agree with the suggestion of going to Tikal in Flores as you'll be in Guatemala already.
Sure enough you can fly from Flores to Caye Caulker via the Belize Int'l Airport on Tropic Air or Maya Island Air.
Flores to Belize Int'l cost US$ 88/person
Belize Int'l to Caye Caulker cos about
US$47 / person.
If you need any assistance with this you can contact [email protected]