You are being scammed (bummer...) What you are seeking to do is celebrate a private committment to one another, an event that has no legal significance to the country of Belize and therefore no paperwork of any kind could possibly be required. As you state, you are already married.
You can structure your event any way you want. The only reason you would need to pay anyone is if you want to hire an event planner to arrange things like a location, a caterer (who of course you would need to pay), etc.
My advice is to find a nice accommodation that has the facilities to locate an event and do it yourself. If you have a lot of people coming from out of town, I am sure the manager at Banana Beach or wherever you choose to stay would be delighted to organize a private dinner for you at no charge (other than the cost of the dinner.) If you want to do something on the beach, the Belikan distributor offers free tents. Most of the restaurants can cater if you want to do a meal under a tent (rather than at the hotel restaurant.)
A lady named Susan Eiley catered our recent birthday party and the food was fabulous and the price great too. I don't know how to reach her but our manager, Nellie Gomez, does. You can email her at: [email protected]
I don't know if Nellie does event planning for a fee or not but she did a good job.
I have heard that Victoria House does a great job on this too...there are lots of places who can put together whatever kind of event you want. But you surely don't need to pay anyone for paperwork.
It bothers me to hear when tourists are being scammed. Maybe the person you dealt with did not understand that you are already married? I'd like to think it was confusion rather than greed.