@YS19
What type of extra service are you looking for?
Just by way of information, if it is a long distance add-on you're looking for there are several options for use when calling from Canada.
If it's a data add-on, there are a couple of options, but remember they expire after 30 days whether they were used or not.
If it's a US roaming add-on for use while traveling in the US, those are only good for 15 days from the time of purchase, so plan accordingly.
