Its probably not as bad to Canada as you fear shipping 1st class.
If it is, just ask your buyer to help you. Everybody in Canada knows the score.
They're all too aware of the score.
If the postage is high and they expect you to stick to the original deal?
They are trying to take advantage of you.
Because they --all-- know the score.
Meanwhile the flat rate envelopes are the very cheapest domestically, by far.
The real trick is to cut cardboard the size of the envelope and get some cheap zipties.
(they have zipties in the dollar store now)
Ziptie your tools to the cardboard and you can mail a tremendous amount of weight in an envelope!
Tying to cardboard is how they ship new tools around the world, and about a million other kinds of products as well. Look in any store, anywhere.
5 bucks and change, insured, tracked, verified shipping.
Beat that.
yours Scott