Ames, I really don’t know what you do, but I think you can relate to this example: Suppose you sold something on Ebay for $250. Would you give $25 to maaser? No, because the item cost you (let’s say) $100 to buy. So do you give $15 to maaser? No, because it cost you $10 to ship it to the buyer. So your income, less (related) expenses is $140. In this case, you would give $14 to maaser.
As far as what constitutes a related business expense, I would consult a Rov (e.g. the childcare example from anon). But cost of goods sold and shipping? Brainless.