Sellers can charge what they want, it is displayed on the listing.
When it comes to combined shipping, or if you think there is a way that a seller can ship something to you for less (like using a flat rate box), it is always best to message a seller before buying the item to inquire about what combined shipping discount they can offer you.
I sell on eBay, so I have a good indication of what it costs to ship within Canada. I also know what flat rate boxes cost. Etc. When I purchase an item, I don't haggle on shipping. There are multiple cases where I believe the seller either overpaid for shipping by not taking advantage of the most efficient prices (eBay discounted labels, flat rate boxes, etc), overcharged, or included expensive security features that weren't needed for the type and cost of purchase (paying an extra $5-$10 for insurance+signature confirmation on a cheap item for example). With that said, I don't message the seller and ask for a discount, or ask them to change their policy. I usually look at the shipping price and the BIN price, if both of those combined are what I value the item at, I buy it.
The other thing is, a lot of time buyers simply don't understand what shipping costs. Most recently, I had an item that used flat rate shipping at $19.99. In this scenario, I would lose money on almost every location, and when you account for supplies and fees, probably break even if it is a regional order. The item was too big for a small or x-small flat rate box. I had a buyer message me with the "if shipping wasn't so expensive! routine". I looked up their actual location, and it would cost me nearly $40 to ship, either by printing a label or using a large flat rate box. I don't believe they were trying to be deceptive, but it shows you that some buyers have a disconnect with shipping costs to the point that they were getting shipping at 50 percent off, and they still felt it was too expensive.
There are also other factors like labels, fees, insurance, and supplies. I am preparing to list an item that requires a double layered box. The box and bubble wrap alone will cost nearly $10-$15. Sure, I could recycle a paper thin Amazon box and use newspaper as padding, but it won't get there safely if I do that. When you pay a seller whatever their prescribed shipping rate is, they only get about 80 percent of that in their pocket because of fees. While I do not always account for that portion when choosing a shipping rate, I would not blame a seller who does.
Ultimately, it's possible that the seller wasn't happy with the auction price and is refusing to give you the best possible rate to use shipping as a means to recoup some money, but you also have to consider all the factors that go into the shipping rate along with the disconnect that a lot of buyers have about the real out-of-pocket cost to ship an item (not just the label).
