If you're staying somewhere else in country first and want to send your gear on ahead you can probably make arrangements with Belizean Shores. Otherwise you'll probably pass out when you see the cost of shipping from outside the country. Nevermind the fact that you'll have to deal with customs and all kinds of other issues.
Personally I would not like to be separated from my dive gear. It is risky enough just having it put in the cargo hold of the same plane I am on. As someone else mentioned, if you are traveling inland prior to coming to the island you might want to ship it over on Tropic Air and have Belizean Shores pick it up for you. Even that has its risks.