How bulky is the item?
The big difference between GPS and USPS/CanadaPost shipping is that the service charges are different. This also applies to UPS/FedEx shipping.
The Duty you are charged is the same. Note, however, that inexpensive (under $100 or so) and small items are often bypassed by CBSA on the basis that the amount collected will be less than the labour costs of collecting.
The sales taxes you are charged are the same. Five percent to 17% of the declared value depending on your province.
GSP charges about $5 service charge.
Canada Post charges about $10.
Courier's 'customs brokerage' charges start around $25 and go up with the value of the item.
If your seller is having difficulty with GSP, have him consider using the postal system. Fun fact, if you live in a remote or rural area, there is a good chance that the courier will subcontract delivery to Canada Post. Which is generally what the GSP does.