I call 888-294-6804. After entering my Jetpack phone number, I use option to pay by credit card. They will first say minimum $15 payment, but offer option for other amount. Using this option, I pay $5.
Evidently their system takes time to update, and I am thinking I will be able to buy multiple months for $15, due to my recent experience.
I had an expiration date of 30 Oct. I paid $5 on 28 Oct. Called back next day and still had expiration date of 30 Oct. So I thought maybe I did need to make $15 payment. So I made two more $5 payments. I called back on 1 Nov and had expiration date of 30 Nov. Thought ok, $10 to keep active was still ok. On 28 Nov I made $5 payment. Today I called and now have an expiration date of 7 Mar 2017, and a balance of $5.
I am not real happy about this payment process, but worth putting up with for the unlimited 3G data.
Hope it lasts for awhile.