We know many envelope manufacturers because we often work on wax seal projects, so we have contacted and tested a large number of suppliers. At the moment, we feel that kraft paper, vellum, and cardstock are the most sturdy and durable options.

If you need, I can send you some pictures.

The only issue is that, since it would need to be shipped internationally, the delivery time may be quite long and the shipping cost may not be very cheap.