You can create an inventory item code called "CUSTOMER-DELIVERY-CHARGES", and do a bulk Goods Received Note at ZERO price, and when issuing invoices to customer, just modify the price accordingly.

As there's zero costs for delivery charges, hence all delivery charges will boost up the GROSS MARGIN. When third party courier companies are being paid, this could be charged to a GL Code that is categorized under "Cost of Sales / Sales Expenses", by doing so, it will keep your Gross Profit intact. If you categorize the GL Code under normal expenses, your gross profit will increase, but net profit remain the same.