Line Items in forms

I am looking for some advice on how to deal with data gathering flows that include dynamic list types of information like line items on an invoice.

Specifically how do I model this from the workflow data perspective and then from a user interface forms perspective.

I have a task that waits while an external process add additional line items to the workflow data until a specific time has elapsed. At which point a back office user will nominate a particular line item for the flow to complete.

Any resources, advice, etc will be greatly appreciated.