This can happen for a couple reasons. One is that your shipping address was entered in a way that doesn't match the USPS system records. Our shipping engine notifies us if this is the case and we'll contact you before shipping to try to prevent this from happening. If we make a mistake in the process, we can re-ship it to you at no additional charge.However, if you tell us to ship it to an address even though it doesn't match the USPS system, and it ends up returned to sender, then we will ask you to pay the shipping cost a second time to re-send it to you. We can invoice you through our website for an easy payment. Alternatively we can issue a refund for the order minus the cost of the original shipping, if you'd prefer that.