File upload on invoice tab

Would you kindly make two adjustments to our PW system? Currently, users raise Ad Hoc purchase orders and attach files or invoices to it. They are currently automatically linked to the purchase order page, but I would prefer it if they were also uploaded automatically to the invoice tab. In certain cases, we have to download all of the files from the purchase order tab and upload them to the invoice tab in order to finish the invoice.Would be much appreciated if you could give me price of how much will cost this amendment