That's not the solution I'm looking for.  I'm trying to condense the report down to a single line item per customer ID, while still including the average time to pay for all invoices during that time period.  Reducing the number of columns displayed is still returning a line item per transaction, which is more information than I need for the report.  The "Average Days to Pay Summary" report is useless, since it calculates the customer's lifetime average, regardless of what date range I set for "Paid Through."