The GSP is made up of the Canadian duty, the Canadian sales taxes, and a ~$5 service fee.
The $90 quote sounds very high for that.
I suspect your seller has chosen an expensive shipping service to deliver the coasters to the GSP plant.
From the plants, shipments are palletted and shipped together to the appropriate countries, where they are passed to a local shipper, usually Canada Post.
How much is the seller charging for domestic shipping?
Shipping to the European Union?
Have you contacted the seller to ask if she will remove the GSP or why she thinks the charges are so high?
We don't see many UK/GSP questions, but US sellers are notorious for using USPS Priority shipping instead of the cheaper, tracked and Delivery Confirmed First Class Parcel International which does the same job.