Although the products are not physical items, as you say, you can set them up in Pastel as though they are physical items, so that the system can generate the Cost of Sales value when you sell them. The system should not reflect a quantity on hand, as you would be purchasing the same quantity from your supplier that you are selling to your customers.

The supplier and customer invoices cannot be generated simultaneously (Pastel cannot do that), but you can process a supplier invoice, update it, and then immediately process a customer invoice.