Do I need to be weary about selling an amp and shipping to a PO Box Box?

UPS store instead.

It may be possible to have it held at a ups store on the us side of the border. Or at a similarly located Fed-Ex depot.

I used to force this ’hold at depot, adult signature required’ scenario, in countries where product would go stolen nor mis-delivered, all the time.

Like Indonesia, or..the eastern block (15 years ago). Or anything outside of the major cities in Russia. And so on.

The deal is they have to go to the depot and show their ID, and their ID must match the name on the parcel.

I have shipped over ten thousand individual mid-high value parcels all over the world. ($200-$500-$3k- etc). I stopped making mistakes long ago....

Is Pay Pal “secure funds”?

Once you get squared on receiving payment , instead of shipping to a PO Box or worrying about a UPS or FEDex store discuss with him/her shipping to one of the parcel storage companies near the border in Washington State (Blaine or wherever) .You ship via UPS or FEDEX to the parcel company's address with the person's name referenced . Recipient crosses the border  and pics it up at whichever company's secure facility.  It is then up to the person to declare with customs as they cross back over and pay whatever in duties.

This link is an example of one so you can see what they offer and how it works http://www.packagereceivingblaine.com/
I've used a similar company in Fargo on occasion with a  friend who lives in Winnipeg .  works fine

I sold a CD player to a guy in Canada all went well. He told me to ship to a post office where he picks up items and cross the border few time a month. Why he does this I have no idea, maybe a Canadian knows. I almost bought a turntable from Canada but after all the custom rules and inspection I got nervous and decided it wasn't for me.