Discrepancies between Purchase Confirmation Certificates and Export Declarations (including free-of-charge items)

We are exporting cosmetics purchased from a domestic supplier. I have an inquiry regarding cases where we receive some products free of charge from the supplier and export them along with the products purchased for a fee. The free-of-charge items include parts of the main products, samples, and promotional gifts. 1. I understand that even for products received free of charge, the unit price must be entered on the Export Declaration for the export to be processed. I have heard that if items specified as free samples are listed as 0 KRW on the export declaration, it may cause customs clearance issues in the importing country. In this case, I am wondering if it is permissible to arbitrarily adjust and enter a unit price for the free-of-charge items. 2. I understand that when issuing a Purchase Confirmation Certificate to a supplier, it can only be issued for goods purchased for a fee. If free-of-charge items with arbitrarily assigned unit prices (as mentioned in question 1) are included in the Export Declaration, discrepancies in the presence of products, quantities, and unit prices will arise between the Purchase Confirmation Certificate issued to the supplier and the Export Declaration. Please advise if this will cause any issues.