How does billing work?
All accounts are set up to be billed on the 5th of every month. If you sign up before or after the 5th you will be charged the prorated difference from that day to the 5th.
If today is the 15th and you are signing up your 1st site with hyper optimized speed and transactional email that would be $37.50 per month. Today you would be charged the prorated amount from the 15th → 4th = $23.75 and then on the 5th you will be charged the full amount of $37.50.
In this example the per day cost is $1.25 ($37.50/30 days). The difference from the 15th to the 4th is 19 days (we use 30 days). 19 * $1.25