NOTE: iPhoto can no longer submit orders for print products. The solution below only works with the Mac OS X/macOS app Photos.
As posted in these threads, there seems to be an occasional issue with address validation that fails when the letter O follows the letter P (regardless of whether there are other characters between them).
For instance, the address
123 Park Avenue, Montreal, QC, W9W 9W9
would be ok, but
123 Parkour Avenue, Montreal, QC, W9W 9W9
In my case, since both the city and province contained the letter O, I removed the letter O from the street address, I removed the whole city name and replaced it with a single letter, and then changed the province to one without the letter O. My address went from something like this
123 Parkour Avenue, Waterloo, ON, W9W 9W9
123 Parkur Avenue, W, BC, W9W 9W9
That let the process continue and (after pressing Ignore on the suggested address dialog), I was taken to the final Place Order dialog.* On the phone with Apple, they also mentioned the possibility of changing the address even at that final step with the down arrow indicated in the image:
Photos froze sometimes, especially at the suggested address dialog (the one where you press Ignore), so this may take a couple of tries.
*There's a chance that you might be able to successfully place the order here, avoiding the final address change. Your postal code, combined with the (hopefully similar-enough-to-the-real-one) street address should be enough for the postal service to figure out the phony city and province. However, since print products aren't cheap, you may not want to take that risk.