Hi everyone,
(please note that this only works if you have a few items, otherwise very time consuming)
The only way that I can think of doing this is go to your post office website, and pick a zip/postal code for each state/province (the furthest city/town away from you) get the postal rates for several weight ranges eg.) 1-3 kg and 0 - 100 grams (the weights that you most often ship or think you will ship).
step 2 set up a spreadsheet with columns indicating the shipping level, the state/province and the various weights you tested. Then divide the shipping costs by the price of the item. Then make a gain/loss columns and do the math. It worked for me and you can put in a price range.
step 3 input these amounts into your spreadsheet.
step 4: set up all the necessary weight ranges and prices in presta shop.
Unfortunately, this is not the best solution, but the best that I could come up with to use percentage based shipping. You should come out even on the shipping costs most of the time if you are trying to match what you pay for your customers
Eg.
price on canada post website =15.96 (regular, and expidited)
price on canada post website = 23.04 (xpresspost)
percentage = 15.96/40 = 39.9