When adding a new Storage Account you have the option to set up each customer for Auto-Pay.
You will check off "Yes" next to Auto-Pay:
Once checked off, a new window will appear to add the Credit Card or select "Added cards" on the left hand side of this window to choose a current card on file. (see image below)
Once a card is added you can choose the "Charge Date".
For example: If you were to choose 01-01-2018, the customer will be charges on the 1st of each month.
If you check off the "Charge Today" check box - the customer will be billed starting today.