You should not be charged duty or tax on items under $20. If you have been, you can call the CBSA number on the back of your invoice and reclaim the money charged in error.
But the Global Shipping Program has only two beneficiaries. The seller gets a tiny reduction in fees for using it, not enough to make any difference to your costs. The big beneficiary is Pitney-Bowes who are raking in those fees that are the basis for your complaint.
At this point, your best strategy is to ask, BEFORE BIDDING, if the seller plans to use this stupid program. If he is, explain why you will not be bidding. Most sellers don't know that it increases the costs of their customer. If you do win something and the GSP shows up, you can ask the seller for a Mutual Cancellation because of the unexpected additional costs. If he agrees, leave positive feedback, it was nice of him.