@gerdgoebel 

If you donate inventory items to charity, just make an inventory adjustment and assign your charitable contribution expense account as the 'Inventory adjustment account'.

Or, if you want an invoice showing the donation, create a $0 invoice for the donated item(s).  The invoice won't book any income but will book the reduction in inventory and the item's cost to COGS.  You can leave it like that or you can then create a journal entry to move the expense from COGS to Charitable Contribution Expense - debit Charitable Contribution Expense, credit COGS.