Best Approach for Variable Amount Utility or Service Bills

There are various types of PO and Voucher Contracts - it appears that you define an agreement, then generate child PO or Voucher with smaller amounts.

In this use case we receive variable amount invoices for "things that have already happened that we didn't explicitly order" for Utilities (e.g., Telephone Service) or Services (e.g., cleaning a grill). These amounts can go up and down by month. We currently just enter these as disconnected transactions as needed by hand.

Is there a correct way to use PO Contracts, Voucher Contracts, or some other object to handle this use case.