Do this in a saved Query, you can name it qryCalculateCustomerAging.
InvoiceNumber
ShippedDate
0-30 days
Current: IIf(DateDiff("d",[ShippedDate],Date())<=30,([TotalInvoice]-[TotalPaid]),0)
31-60 days
Over30: IIf(DateDiff("d",[ShippedDate],Date()) Between 31 And...