Best answer by Allan M
Once you have the easyroam add-on on your account, once you use your service in the US you will get charged $8 and be able to use your service for 24 hours. If you are still using your service you will simply get charged another $8 for another 24 hours. Rinse and repeat until you get back to Canada.
It basically just $8/day but instead of screwing you over if you used it at 11:59, they do $8/24 hours so that you can get the most out of it.