Let's start with why sellers refuse to ship to the most secure method of getting the mail.
They are in the USA and prefer to use UPS or other couriers who, for much the same cost as USPS will pick up, insure, track and deliver parcels.
However, couriers cannot deliver to PO boxes, so they (should) state in their listings that :
a) they deliver by courier
b) that they cannot ship to PO boxes.
Some don't think this through and do neither.
So.
How to avoid these sellers?
Read the listing of US sellers, including the part about shipping, and avoid those who ship by courier.*
However.
Many lazy or naive sellers will copy, holus bolus, the Terms of Sale of other sellers. Even though they will be shipping by mail, they copy the part about not shipping to mailboxes.
Just to make this sillier, it appears that the norm in the USA is to have the mailbox at the bottom of the driveway, where anyone can open it. But those are apparently just fine, while a PO box, which requires a key or for parcels photo ID and a signature, is not acceptable.
Basically your seller is either dumb, a copycat , or shipping in the most expensive* way possible. Avoid such sellers.
*The "howls of complaint about UPS" thread is well over 5000 posts long and continued for many years.