Purchase order:total issue

In purchase order, i have kept an additional filed called “Packing charges” , total purchase order values is ex:5000 Rs.

Packing charges (is enterd by po user ) Rs.500. so total
5000+500=5500 i need to update this amount in grand total,but it is not adding the additional field.

It shows grand total is 5000 Only

As you have added custom field you need to make changes in code too!

Yes, i should do the code ,but where should i need to do the code change.
Could you please explain the code block please

Any Idea for this scenario

Instead of making changes to code, other work around is, you can add packing charges as TAX or create a separate item code and add it to PO.