Yes, Steve, get copies of the sale close, etc., and other proof. Have a copy sent to you and also have the sender enclose a copy with the shipment. WHY insure for $900? The insurer (the PO or shipping agency) would ask for proof of price, and would pay out only $312 anyway in case of loss.
In the meantime, Customs values at insured-declared value and assesses on the $900. I had that happen to me once and it was much work to get a re-evaluation and refund (On an item sold on Ebay for $100plus and the seller put $500 value on it).
Note about United Parcel Service- They add a brokerage fee here (Alberta) which is not really required since Customs lets you broker the import yourself. Purolater, on the other hand, has a very small fee ($3.00 or so). I know Purolater is not convenient for some; depends on their depots I guess.
What Bull Sables says doesn't apply since the duty was eliminated long ago, but you are liable for the taxes, GST and Prov., parts or not.