Note, extending your membership may vary depending on your account.
Editor's Notes & Price Research
ShopRunner is a membership fee based service that offers unlimited free 2-day shipping and free returns on select partnered stores. Offer valid while promotion last. Please refer to the forum thread for additional details - Discombobulated
No t-mobile service necessary.
If you are a new member proceed to create your account here and you will be rewarded with 1 free year of Shoprunner.
If you already have a shoprunner account (with an expired subscription) just type in your current details even though it is asking you to make a new account. 1 free year will be tacked on.
YMMV Some people with active memberships also report being able to simply extend memberships using the above method.
Alternatively, you can make a "new" email account by placing a period (.) in your email username to get a free year on your "new" account.