Adding a "Price paid" field in the Pricing area of the Purchase Order Line Details form-- and/or making the Net price per copy box accept text input-- would be helpful for invoices/receipts that only include the price paid.

Currently one needs to calculate the Discount (amount) [or Discount (%)], which may also include looking up [current] List price before subtracting the amount paid to calculate the discount.

(Speaking of current List price, if this could somehow be "grabbed" to automatically populate the list price field, rather than needing to look up the information and manually input the amount, that could be very helpful as well.)