Are you the buyer or the seller?
If you are the seller, remember the old computer meme GIGO.
Bad inputs lead to bad outputs.
Do your dimensions and weights match the shipping service you are using?
Are you allowing for eBay/PP fees and for packaging costs?
Do you use PP labels, many of which have built in discounts?
If you are advertising higher than necessary shipping, you can still reimburse your buyers using PP's Send Money service. The refund need not be the exact amount of the postage overage, since you also have fuel fees, sales tax and PP/eBay fees in your postage cost.
If you are the buyer, is this a GSP complaint?
The only "additional" charge on GSP items is the ~$5 service charge. The duty and sales taxes are Canadian law and are remitted to the Canadian government.
Because Canadian public servants are smart, the workers at CBSA don't usually bother assessing incoming parcels that are valued under ~$100CDN, because they have noticed that it makes no sense to collect less in duty and tax than they spend in labour costs.
Couriers (like Pitney Bowes who run the GSP) have no such choice. They are legally required to assess and collect duty and sales taxes on any item valued over $20CDN(about $15USD).
Your choices are:
Don't buy anything valued under $100USD that is sold through the GSP.
Ask the seller if he is willing to set up a private listing for you that uses USPS First Class International Parcel shipping, which -- and be sure to tell him this-- includes Confirmation of Delivery.
Note that USFS/FCIP is not necessarily cheaper. And that purchases over $100 are much more likely to be assessed by CBSA and appropriate duty and sales tax collected on your doorstep.
Buy Canadian.
