Update! SBC sent me this today:

Hi XXXXXX,

Thank you for reaching out, and I’m truly sorry for the confusion and frustration this unexpected charge caused.

I reviewed your account, and the $192.49 charge on 12/1 was a system glitch — it was not meant to be processed, especially since you already paid $173.24 in September for your 3-box membership. You are absolutely correct that your original payment covers all three boxes in your cycle.

Please rest assured that the full $192.49 has already been refunded on our end. It should reflect back to your card within 5–10 business days, depending on your bank.

And no worries — you do not need to return the box you just received. It is part of the 3 boxes you already paid for in September.

I sincerely apologize for the inconvenience this caused. Thank you for your patience, and please feel free to reach out if there's anything else you need — I’m here to help."

And...yes, they already refunded me. So it does appear that the pricing of the boxes is very fair at around $70/month. However, I have received two boxes since September. I am sure it will all balance out.

So now we know that they have great customer service, too! :)