What is the FASTEST most secure way to send an important document from Belize City to the USA?
FedEx. You can even drop it off at your nearest Tropic Air Depot, as they are an agent of FedEx. There is a fedex desk in the Tropic Cargo office. Takes about 3 days. Reliable. Works US to Belize also.
You can use Docusign via internet to send legal documents back and forth from US. Real Estate offices use all the time for legal documents.
MS service is an International service and you send it through the POst Office. It is $50 BZ dollars but it will arrive within 5 business days, gauranteed. I have used it many times and very happy with it. You can read about it online.