sum if value in cell in a certain column starts with number

Hi,

I am looking for a formula which I need to retrieve values in sheet PO List, whenever the content of a cell in column Invoice number in the source sheet ERP starts with a numeric number
and shows in column N the respective PO number.
Attached Excel for illustration my problem.

To retrieve all the amounts in the source sheet which start numerically I tried to use this formula but it is not working.
I was trying the same with a Sumifs formula to no avail either.

SUMPRODUCT(ISNUMBER(--(LEFT(ERP!$P$1:$P$60000,1))),ERP!$N$1:$N$60000,D8,ERP!$A$1:$A$60000,N5,ERP!$E$1:$E$60000)


Many thanks for your help in advance
Philip