On App Center documentation I found :
The Start Date of the Commitment Period
When you sign up for a commitment period, you must pay for the remainder of the current month plus
the selected commitment period.
For example, if you sign up for 10,000 emails with a three months commitment on August 15th, you
must pay for the remaining 16 days of August plus the three months of September, October, and November.
The total number of emails available in that period is 35,161 emails — equivalent to the remaining 16
days of August — 5,161 emails, plus 10,000 emails per month for September, October, and November.
The following 3 months will be at the normal price and volume. The renewal period starts on
December 1st — this is also known as the renewal date.
If you only want to pay for the commitment period, you must sign up on the last day of the previous
month. For example, if you only want to pay for September, October, and November, you must sign up on August 31.
My recommendation is to contact a sale guy from your region.