There are mail forwarding services. They have a bunch of PO boxes, give you an address consisting of the PO box address and a made-up prefix eg Flat one millio

There are mail forwarding services. They have a bunch of PO boxes, give you an address consisting of the PO box address and a made-up prefix eg Flat one million. They have a web interface which you use to view received mail, trash the mail, forward the mail to a specified address, select forwarding option eg 1st class mail, 2nd class. When you move, add your new forwarding address.