Procure to pay and order to cash describe the same commercial transaction from opposite sides. Your purchase order is somebody's sales order. Your goods receipt is their delivery. Your invoice processing is their collections problem. Seeing the symmetry explains a great deal about why suppliers behave as they do, and it makes both processes easier to design.
The mirror, step by step
Their quote is your requisition input. Their sales order is your purchase order. Their despatch is your goods receipt. Their invoice is your invoice. Their receivables chasing is your payables queue. Their aged debt report is your aged creditors report. Every step you find frustrating on one side is somebody experiencing the reverse on the other.
Why suppliers chase the way they do
A supplier chasing an invoice is trying to answer one question: when will this be paid. Their own process needs a date to forecast cash. Providing that date proactively, through a remittance advice or a clear expected payment date, removes most chasing, and it costs you nothing because you already know the answer.
What each side can do for the other
As a buyer: give order references, tell suppliers one billing address, and pay to terms. As a seller: quote the customer's order reference, invoice promptly and accurately, and send statements. Every one of those reduces work on both sides, and organisations that do them well on the sell side often have not thought to do them on the buy side.
Questions people ask about procure to pay and order to cash
Should the same system handle both?
Some suites do. In practice the two are usually owned by different teams with different priorities, and a shared system helps less than shared conventions do.
Which is harder?
They have different difficulties. Buying is about control and matching; selling is about collection and cash. Both are dominated by the quality of the reference data on the documents.
Does understanding the mirror help?
It makes supplier behaviour predictable, which makes disputes shorter. Knowing what their process needs from you is usually the fastest way to get what you need from them.