You could do this with a Pivot query.
Create a new query (Sub Query 1). Leave blank for now
Add another Sub Query (Sub Query 2)
Add the following tables
Link Customer to InvHead on Company and Customer Number
Link InvcHead to InvcDtl on Company and Invoice number
Add the following fields
Calculated Fields
InvMonth DatePart(Month,InvcHead.InvoiceDate)
InvYear DatePart(Year,InvcHead.InvoiceDate)
YearTotal Sum(InvcDtl.ExtPrice)OVER(PARTITION BY Customer.CustID,InvYear)
Go back to Sub Query 1 and add Sub Query 2
Right click on Sub Query 2 and set Pivot
Set the Pivot field to InvMonth
Add Months 01 thru 12 (Need to have the leading 0)
Set your Aggregate Formula to InvDtl_ExtPrice
Add your field to Sub Query 1
Run your Query