There are 2 scenarios where you could be charged twice:

1. When you ship your box and when you are renewed for a credit to redeem the following month.
2. At the beginning of the month when you ship your box, if you purchase any add-ons and decide to pay using cash over available credits, you will be charged at the time you ship your box. At the end of the month, if your membership is set to renew, you will be charged for a credit to redeem the following month on your renewal date.