If you won't use Tor (you should), then at least use a clean install of a new browser with a fresh VPN exit node so as to minimize being tracked by tracking cookies from Facebook, Google, etc.

The extension you want is uBlock Origin, not uBlock. The two are different, and the latter allows through "approved" ads and trackers.

This is probably the hardest step, and also very dependent on what country you live in. Generally speaking, PO Boxes or similar have to be registered in your own name, and you need to provide ID to be allowed to open them or sometimes to access them. Even so, this is still a good step, as it breaks the link between your real name and the retailer, and an attacker would have to be very determined to go about de-anonymizing your PO Box (provided you aren't careless and don't advertise it elsewhere). Other options include general delivery or poste restante, delivering to a work address (but then it either has your real name, or you need to have a receptionist who will look out for a package with the fake name on it), or sometimes you can arrange delivery to a store or similar drop off location which won't require ID to pick up.