You can find this section in product configuration in Settings → Products & Services → Your Product → General Settings → Enable ProRata Billing
To enable pro-rata billing for particular product/service go to product configuration (Settings → Products & Services → [Your Product]) and in the General tab click on "Enable ProRata billing"
Setting "Disabled, try to bill for entire cycle" under this option will issue annual invoices on the month that customer purchased service, for example if the customer purchased Annual package on February, he/she will get next invoice exactly on February next year.
For example if set to "15" and the customer purchased service on 17th of the month - he/she will be charged for remaining days in the current month + entire next month. To disable this feature set its value to "0"
Example 1:
Product settings:
Prorata day: 1
Monthly price: $10
Customer purchased product on 13th of month:
After month of usage next invoice will look like as below:
Example 2:
Product settings:
Prorata day: 1
Charge Next month: 10
Monthly price: $10
Customer purchased product on the 13th of the month: