A tip - the adhesive labels that will run through a computer printer will, when applied to the back of a print, add both strength and durability, while offering readability for both the postal service and the recipient.
I just finished a 500 sheet box of Kodak Polycontrast III postcard RC paper that Dennis (dances with clouds here on APUG) gave me.
So for the next round, I'll be cutting 8x10 sheets of Ilford MGIV RC glossy paper into three 4x6 sheets.
In Canada, we don't get a postage break on postcards, but we can send a much larger card at the letter rate (domestic 63 cents now but going up to 85 cents at the end of March, when stamps are bought in packs).
In the US, however, there is a really cheap domestic (32 cents) rate for 4x6 cards, and the $1.10 rate for international 4x6 cards is much cheaper than Canada Post's rate. As I can just about see the US from my kitchen window (there are a few trees in the way) and as I'm about 15-25 minutes drive away from a small border crossing, I print my cards to the 4x6 US standard and use the US postal service for US and international cards.
I will be interested to see what the costs for sending mail to the US and internationally will change to after March. I would suggest though that if the costs concern you (as they do me) then you can always limit the number of cards you sign up for. I find 24 to be a good number for me.