Accounts payable has got noting to do with purchase of goods and services, it's work is to pay the eligable invoices.
End to End Process of AP - Process the vendor invoices into system - Work and resolve the discripencies of the invoices (if any) - Make the payments to the invoices as per the agreement - Resolve the issues ( if any ) regarding the invoice payments
For material/Goods Purchases, the Po is required. In the Po, QTY, Product Name, Where is delivery price, Terms and Conditions. In the po there is no matter of final amount. It is just the approximate amount.