I'm struggling to see any capability so far within https://schema.org/Invoice or https://schema.org/Order to support itemised receipts that include line items which specify quantity, product description / GTIN / SKU and a https://schema.org/PriceSpecification per line item.

Have I overlooked something?

We are interested in semantic modelling of digital receipts and logically a receipt is an invoice in which the https://schema.org/paymentStatus is https://schema.org/PaymentComplete (unless the card transaction failed etc., e.g. https://schema.org/PaymentDeclined)

We're currently looking into existing specifications for machine-readable receipts and may be able to contribute a proposal for enhancing schema:Invoice to support itemised receipts.