Documentation describes the following value for Charge for Subscription:
End of Month - the customer is billed by the end of each month.
However, in Provider Control Panel option End of Month is missing.
The value End of month for Charge for Subscription option was dropped in PBA-E 5.1. The documentation is to be corrected in scope of a bug #PBA-58944.
In order to charge customers on the last day of month, please, refer to the article "How to set Day of Statement to the last day of month?".