Use a Pivot table to work out average order value of invoices

I have sales invoice/credit note data and I want to find out the average order value for any given month. My report has multiple lines per invoice, as some invoices may have more than one product. I have tried to work out the value using a pivot table but the results are skewed due to the count of the multiple lines on certain invoices.

I'd be glad of any help on what fields or functions to choose in the pivot table as I can't seem to get the answer I'm looking for.

Thanks!