To add rounding off/Shipping charges, first let's start by creating accounts.
From Charts of accounts --> New Account, add the name of the account-"Rounding off" under the head "Charge against sale/Purchase":
If you want to add it in the invoice window- create it against Sales,
If it is to be added in purchase window- create it against Purchase.
Once you have created the account, in the Invoice window from "Add other charges", select the account you want to add and also the value against it.
That value will be added to the Invoice/Purchase bill. If you want to subtract it from the Invoice/Bill, add a "-" sign before the value.
In this manner, you can add shipping charges, discount, round off or any other charges in Invoices/Bill.