I'm in the same situation. From my investigation it appears that this is the procedure:
1. Purchase your annual sub on ebay. You select the plan & carrier and can specify if it's a renewal (no new sim card). You can elect to recieve the sim card and not use it. After the purchase you will be emailed an activation code and the sim card will be sent via USPS. Don't purchase the plan more than 30 days before the current one expires.
2. Log in to your Redpocket account and there is a selection for entering the activation code. It is said that if it's the same carrier & plan it will add on to your existing expiration date. If you are adding a different plan, the expiration will be 365 days from the day you enter the code. In my case I have purchased a plan, which has a smaller data allowance, and will add the code a day or two before the current one expires (I'll lose a day or two).
I made my ebay purchase a couple of weeks ago but haven't added the new activation yet, so keep that in mind. There's always the online chat or phone call with customer support route.
Edited to add link:
https://help.redpocket.com/how-does...month-plans-3-month-6-month-12-month-24-month
And the ebay activation code *might* be good for 90 days.