Invoices
A price exists once it is documented. A sale without an invoice is a sentence: neither a gallery, nor a foundation, nor a buyer who wants to resell the work a decade from now can confirm what it cost.
An invoice is the first link in your practice's price record and a tax
document at the same time. In ArtFond it is assembled from the work's own
catalogue entry and your details, so nothing is filled in twice.