You may have walk in customers that pay cash. You do not need to enter a sales order or generate an invoice but rather ship the product, receive payment and generate a 'Sales Receipt' in QuickBooks. To set accomplish this set up a term with a type of 'Cash'. Next create a ship doc and select the term. You will notice that the Invoice icon is replaced by a 'Sales Receipt Icon'. You will also notice a payment icon on the bottom right of the screen. Click payment icon to enter the customer's payment information.
After completing the payment information, click 'Save' to return back to the ship doc and Save again to commit the transaction to the database. Finally click the 'Sales Receipt Button' to record it in QuickBooks.