How to add postage to a transaction

Marina

Hi, I was wondering if anyone had any idea how I can add postage as an extra category under -Transactions - Add Income area. For example, if I've charged $100 for a pair of earrings but I paid postage of $10 for that sale. Do I just show the Income as $90? I would like to be able to see $100 for the sale less $10 for postage. Hope that makes sense and thanks for your help!