How am I billed?

Billing happens every month based on date you signed up. The total that you're billed is:

Total number of sites X Price per site

For example, let's say you created 13 sites a while back, then your next bill would be as follows (note that the price per site is $12 when you have between 10 and 19 sites):

13 * 12 = $156/month

When you add a site in the middle of the month, the total is prorated according to when you add that site.

For example, lets say you add 2 new sites on the 20th of the month to the already 13 sites you have, and your billing date is on the 1st. The price per site stays the same at $12 but the total will be as follows:

(13 * 12) + (2 * 12 * 20/30) = 156 + 16 = $172