Depending on your envelope size and weight, the price of shipping to Canada and UK can increase. I would first look into USPS and see how much it will cost to send your product to Canada and maybe change your shipping default for Canada, 5$ might not be enough to cover the cost of your product. Also Canada and USA have shipping regulations. I would check out this site and read up about shipping and taxes:
You need to also make sure your product is allowed to be shipped to Canada as there are laws of whats good are allowed in the country.
The same thing goes for the UK. I would check these before deciding to ship internationally. Shipping to UK can create Duty taxes and sometimes your items can be held at the border if this happens you might have customer problems.
Good Luck with the shipping!