text: how many stamps do i need to send a letter to the united states, from canada.? i am in canada (ontario), and i need to send a letter to santi monica in the united states. how many stamps need to go on the envelope? use the canada post website to calculate postage based on your envelope weight and size.  http://www.canadapost.ca/personal/tools/rc/res/bin/rc_res_us-e.asp