Invoice Approval Routing

Can anyone help me with invoice routing for invoices with multiple distribution lines that have different AUs. The invoices are routing by total invoice amount and the AU. Right now its routing by the first AU in line on AP20, regardless of how many AUs are listed. I tried using AP52 to interface these in an unreleased status so we can use a suffix for each line, so they route separatly. But, it isn't possible to interface an unreleased invoice on AP52 with a suffix and the flow is triggered by the invoice being released. I got error messages on AP520 that say: the suffix has to equal zero for an unreleased invoice. If anyone knows how we can trigger the process flow for interfaced invoices it would be a big help?