Some sellers add small amounts to the shipping cost, probably depends on their local shipping rates. I can usually get four copies sent for $8 within Europe.
Yes, as long as I don't consider the item to be too rare, cheap or in-demand.
Usually not.
I add cheap records to my wantlist in hope that the seller has one or more of these in stock. If you go to the list of copies for sale ( https://www.discogs.com/marketplace?release_id=2504692 ) the number of items on your wantlist will appear in the "seller information" column as a link. This way you can see all the wantlist-items the particular seller has in stock. Having a big wantlist will make it easier to find records to add when you're placing an order.
(If the price is good I'll order the copy I was looking at even if the seller doesn't have any of my other wants.)