Coupons aren’t just for the bricks-and-mortar I’m more prone to buy something from an online store if it’s on sale, or used (think Ebay), but here’s a tip my sister taught me about online shopping:
Just after you’ve filled up your online cart, Google the name of the store you want to buy from and “coupon code” in the search engine and start trying codes at checkout. Or check out coupon wikis like Alex’s Coupons, or RetailMeNot.
These sometimes expire quickly, so you might have to try a few, but usually you can come up with one that will give you a discount. My favorite ink and toner supplier always has free shipping, but they also usually have codes out there for 5%, 10% and 15% off, but I’ve noticed that about once a quarter they also do a 20% off coupon code (they announce these on twitter so I watch their updates carefully—often leaving my items in my “shopping cart” for a while until a good code materializes).