T-shirt orders are no longer available for Christmas delivery.
For all other products, *domestic* orders placed by noon (eastern) on the following dates for the respective services should be delivered before 12/25.
Standard Shipping, USPS First Class Mail: Tuesday, 12/20 (although you're really pushing your luck here)
USPS Priority Mail: Wednesday, 12/21
USPS Priority Mail Express: Thursday, 12/22
As for guarantees...we guarantee we'll get your order out the door in accordance with the deadlines imposed by the carriers, as listed above. But since we're not doing the actual delivering, we can't and won't guarantee it'll show up at your door when it should and are therefore not accountable for late deliveries. Those services are pretty reliable though - so you should have faith - but everyone makes mistakes. And snowstorms and stuff.