You can add Invoice Charges in the footer of the invoice, quote or order. Go to Invoicing - Customer Invoices, click on Create Customer Invoice and start creating your invoice.

If you are creating an order or a quote, you can go to Sales Order - Create Order or Quotes - Create Quote.

If a field is inactive (grey) and you are not able to register anything, it is because there is no default account registered under Registry - Chart of Accounts - Default Accounts.

Invoice Charges can also be pre-set as follows:

• If you want a specific charge on all your invoices, add the sum you want to charge under Settings - Purchase Order/Invoicing/Sales Order - Invoice input view. See below.
  
       
• If you always want to have a specific invoice charge on the invoices you create for a specific customer, add the invoice charge to the customer under Registry - Customers - (Select customer) - Additional Information.