The price for your art work should reflect how much time it took to make the piece (paying yourself at a fair hourly rate) as well as the cost of materials. Since they are on canvas and they're fairly small, they should be light and not really cost that much to ship. You could pack one in bubble wrap and weigh/price it out and assume that will be the standard price. You can't be too far off.
As for buying shipping supplies and boxes, it really depends on the volume you do. Personally I recycle boxes from work and from what comes into our house and I know customers appreciate using recycled materials. So, you should be able to save yourself some money there.
I guess my general advice is don't underprice your art work, but try to keep down costs/prices for customers everywhere else!
I think that answers all your questions?
