Timing and alternatives for applying for a Local L/C (Purchase Confirmation) to prove indirect export performance

I am a supplier who has never received certification for indirect export performance before. Since the export process by the company we supplied goods to has not yet taken place, I am curious about the timing for applying for a Purchase Confirmation (Local L/C). For example, I would like to ask assuming a situation where our supply of goods, payment receipt, and issuance of a standard tax invoice occurred several months before the exporter's actual export timing. 1. From when to when can a Purchase Confirmation be applied for? 2. If the exporter says they cannot apply for a Purchase Confirmation, are there other ways to prove indirect export performance? 3. In a case like the example above, where supply and payment occur before the export and a standard tax invoice has already been issued, is it still possible to apply for a Purchase Confirmation and prove indirect export performance?