There are a couple caveats and conditions:
First, the guarantee is that USPS will attempt to deliver the package by 12/24 at the latest. If there is nobody home to receive it, and the package does not fit in the mailbox, we are not responsible for the failure to receive the package in time.
Second, the guarantee is subject to product backorder. As we print on demand, we occasionally run out of blanks in a particular size or style. This is due to two factors -- one, when multiple large orders for single sizes come in at once, the system sometimes allows more purchases than we have inventory for. Two, the printing process we use sometimes generates flawed prints that we aren't proud of, and instead of sending an inferior product we simply print another. The inevitable result is that sometimes, an order is placed for a size that we then run out of before fulfillment is possible. In the event that an item you order becomes backordered before shipping, we will send an e-mail notification as soon as possible. You will have the option to continue with shipment of non-backordered items and ship the backordered item as soon as it becomes available, the option to continue with shipment of non-backordered items and cancel the backordered product, or simply cancel your order all together. This e-mail will be sent within 24 hours of placing your order, at the absolute latest. It's something pretty rare, and definitely something we hate having to do, but just be aware that it is a possibility.
Third, it's always possible some factor outside our control (USPS plane crashes, civil war breaks out, massive blizzard delays shipment, etc.) will prevent a package from actually arriving on 12/24. If this happens, well, we're really sorry, and will happily allow a return for full refund.