We set up a business account and used the business PO box for the return address. But that's because we are a business. IANA/Attorney/Accountant, but generally speaking, all income from the site, whether from user upgrades, advertising, merchandise, etc, is "income" that should be reported on your taxes. Likewise, costs incurred are expenses or losses. For us, it was easier just to pay the $200/yr LLC filing fee and organize as a business. Keeps the paperwork tidy.