If something happens and you run a few days late shipping something, as long as you are up front about the delay its not such a big deal. It only usually becomes a issue for someone if you just stop communication, or the buyer is a impatient dick. About the only exception to this would be if its a time sensitive/gift related buy, and they told you of such when making the purchase. Then it does become a issue and falls on you to make it right if it doesn't get there in time.