I activated the 3G on my daughters through the internet directly over her Ipad. I think we had an issue for availablity when I punched in our user address area. So I put in my home address for billing and my husbands work address for user area and it worked fine. It will charge you every new month unless you remember to go into your settings and unsubscribe. You can purchase it for a month and unsubscribe a few days later, it will still give you coverage for the month you pay.
We use the 3G on long trips and on vacation. It is slower than the regular wi-fi hook up and for me, it isn't worth keeping activated year round when we have 3G at home.