My (PMB) private mailbox provider has told me that they don't care what I do to get my box number on the address, as long as it saves them from having to look me up on their computer to get my box number.
They don't care if the address says: PMB, SUITE, APT, UNIT, SITE, BLDG, FLOOR, SPACE, #, .... You get the picture.
I would say to use whatever syntax that gets the box number on the address line, and gets the mail delivered to you.
I haven't followed the USPS rules for a PMB. I use this format:
my name
99 my street name APT 999
My City, XX 12345-6789
If you have a PO Box then I don't know how one could game the address with an online retailer and still get it delivered.