A Purchase and Sales Invoice (PSI) is an important document that summarises essential information about a business transaction. It includes details of what was bought or sold, the cost of each item, taxes and other charges, plus payments made or received. In some cases, it also includes additional services or goods related to the transaction. PSIs are used by businesses to track and record their transactions, making it easier to match payments with orders and reduce paperwork. The PSI is an important tool for businesses, allowing them to manage their finances efficiently while helping to keep track of any changes in the market or customer needs.