Artsper plays the role of intermediary in the sale, but it cannot under any circumstances be considered the seller.
It is therefore the gallery’s responsibility to issue a sales invoice for the customer, as if the sale had been made at the gallery. It is not necessary to include on the invoice that the sale was completed through Artsper.
The invoice should simply contain the price of the work, delivery costs to be paid by the buyer, and any discounts the buyer may have been given.
The gallery will need to upload the invoice in PDF format after confirming that the work has been dispatched in the "Orders" tab of its gallery space (more information here).