Advice for shipping from Canada to US?

As a Canadian who has shipped to, and from, the USA many times I would recommend you use either the postal service or one of the reputable couriers like UPS etc. Also ask for shipping insurance, it makes the delivery require a signature so no leaving the package on the doorstep. Small cost IMO for what you get, note it wont help if damaged but only lost. But it will make the delivery service treat it differently then regular packages, Ie signatures, duties services, etc.

If using a Courier like UPS I recommend you ask the seller to take it to a UPS store to have packaged and shipped. 

Contrary to the advice to avoid the postal service, I have had good experience receiving packages from Canadian vendors shipped that way including one received just yesterday.  I buy stuff all the time from Canada, really it is not much different from buying from US vendors.  The unknown factor X right now might be a change in the political climate, which we have zero control over.

Have the seller contract with a freight forwarder on the Canadian side and have your amp customs cleared in the US and shipped to your door.  The forwarder has contacts in the US, both for clearance and transportation.  Sometimes it will be the same party who does both.  Of course, you two will have to negotiate the price for all this.   It ain't the cheapest route, but it is the easiest one for you.  

I've had customs issues with USPS shipments from both Canada, Europe and Australia. USPS lost rare Technics SP10 power supply, ended up having to search and purchase another power supply, then lo and behold they somehow discover the original power supply months later. Also have experienced shipments from Canada held in US customs for duty payment. USPS doesn't act as broker, UPS,  Fedex, DHL are brokerage agents.