I've asked a similar request as Justin had in 2013 and Krystal @ting responded with a definitive answer below.
Pre-paying for Estimated Usage
Can I pre-pay for 6 to 12 months for my estimated Ting use so it is unnecessary to pay the credit card bill each month when there are no other charges except ting? How can I do such pre-paying?
Krystal Provencal Yesterday at 23:44
Thank you for reaching out to us! From your email I see that you'd like to know how to setup a way to pre-pay your account, so you don't have to pay your credit card bill monthly. I can definitely give you some advise on this.
You can use a VISA or MASTERCARD debit card instead of a credit card or you can use a pre-paid credit card. You can load whatever amount you'd like on that pre-paid credit card and use that on your account instead.
Aside from those options, there's no way to actually pre-pay for your services. I hope this information helps! If you need anything else, please don't hesitate to let us know. We'll be happy to help!