Timing of Local Purchase Confirmation Issuance and Zero-Rate Application for Services in Advance and Balance Payment Transactions

We are an equipment manufacturer that receives overseas orders and produces goods by procuring services and parts domestically for export purposes. To reduce the financial burden, we intend to issue Local Purchase Confirmations (구매확인서) to apply the zero-rate tax rate, but we have a few questions. 1. Since the purchase amounts for parts are large, transactions are usually conducted in the form of advance and balance payments. 1) In this case, must the Local Purchase Confirmation be issued in the same month the advance payment is made, or can it be issued at the time the supply of goods or services is completed? 2) If it must be issued in the month the advance payment is made, can we issue the Local Purchase Confirmation then and receive the zero-rate tax invoice at the time the actual supply of goods is completed? Alternatively, can we issue the Local Purchase Confirmation at the time the supply of goods is completed and then receive the zero-rate tax invoice? 2. We need to enter into contracts for services required for equipment manufacturing (design and assembly) and pay advance payments. We would like to inquire if it is possible to issue a Local Purchase Confirmation for these services to apply the zero-rate tax rate.