Easy to do, best location for you if you are based on San Pedro would be Altun Ha ruins. Just the two of you or you have other folks with you? You can take the boat from San Pedro or fly into into Belize City and drive up.
You'll need three things-
Permit from Dept of Archeology to have the wedding there. No cost and they allow minister, bride and groom in for free, guests pay regular entry fee of about Bz$5.00pp.
Marriage application, signed and notorized. Law says you have to be in Belize 3 days and can submit your application for marriage to the registry office in Belize City on the 4th business day (not a weekend or local holiday). You can have the paperwork returned and ready for your ceremony by the afternoon of the following business day. License costs US$100.00. My office can facilitate this for you at a cost of US$220.00, which includes the license, notary, etc. The people at the web site you listed below do the same, so you have choices.
Minister to conduct the ceremoney. Catholic priests can not conduct a ceremony unless you have special paperwork from your parish at home. We have other ministers here who do, sectarian or non sectarian. They usually charge about US$150.00.
Photographer? Music? Flowers? Celebratory parties or dinners? What else.
There are other good wedding planners, ask around and talk to them all.
Maya Travel Services