You can generate an invoice from multiple packing slips if you access Sales > Sales Slips or from SAT > Work Slips. Click on Filter and indicate Customer and Status "Pending."
.Select the delivery notes you want. Click on More > Generate > Invoice.
An invoice will be generated with all the delivery notes we have selected.
Invoices cannot be generated from different module delivery notes, so you have to have all the delivery notes in the same module.