Many law firms and IBC formation/offshore service companies, mostly based in Belize City, can provide a Belize mailing (or in some cases even a physical) address for an annual fee, and folks like Quicksilver in San Pedro can provide a P.O. Box and mail forwarding service.

But even if you are QRP (not technically a Belize residency status) or Permanent Resident or even a Belize citizen that doesn't mean you are in a tax-free regime ... except in the very limited sense that income earned outside Belize is not taxed IN Belize. But income earned elsewhere is still taxed based on the tax laws of the country where the income is earned.