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

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.

Weary?  Probably not...leary or wary?...possibly. ; )

So what you do is look up a FedEx (or UPS) store near the border. Then you ship to the FedEx (or UPS) physical store address, in the name of Will Call and name your receiver. There should be no fee for this, and if there is its one of the contractor stores and you just go elsewhere. Actual FedEx and UPS stores do not charge for this. They just check ID, and away it goes. Do this myself all the time as it eliminates any risk of the package being thrown on and off the ruck a few extra times trying to deliver when you're not there, being left outside, etc. Only way to go.

I have used Kinek before, www.kinek.com.

It is not free, usually $5 or $10 depending on the location, but they have more convenient locations and better hours than FedEx or UPS stores.