It's still a puzzle for me on how the renewal process works. Here's my experience using the phone-in process so far. I added $5 each time.
Paid on Nov 13. Service renewal date Dec 14.
Paid on Dec 10. Service renewal date Jan 9. Balance $0. Renewed early to see what happens.
Paid on Dec 22. Service renewal date Jan 9. Balance $5. I don't recall why I did this so early.
Paid on Dec 28. Service renewal date Jan 9. Balance $10. I did this because of the message stating "As of December 29 MyVerizon services will no longer be accessible for your device".
Checked status Jan 10 and message says account expiration date Mar 14. Balance $10. No mention of renewal date.
So far my service is still working. If my service ends early, I will try using one of the eBay sellers services. They will renew service for about $6 a month.
Worst case, I will have my Jetpack reprogrammed by one of the eBay sellers. Typical charge is about $40 and includes two months service.