You'll NEED to get a R/T as very often they ask to see that you have a return leg... not on the Belize end mind you, but the US end ('making the assumption you're coming from the US... not sure how other countries handle it.) Pretty much EVERY time we fly back to the States from here (with a ticket Belize - US - Belize) we are questioned at the counter in Miami/Dallas if we have a return tix. We then show them our residency card and it's ok.

Buy the R/T tix and just make sure the tix is such you change change the return date for a minimal fee...