After trying lots of things that didn't work, this worked for me!  I typed in Box 333 in as the address, and then QBO asked if I wanted to leave it or use their recommended version, which was PO Box 333, which was exactly what they said I could not use when I typed it in myself.  Intuit really needs to fix this...but in the meantime, at least this workaround enabled me to get the correct mailing address in for our employee.