Sip if they under charge you shipping at time of checkout will contact you before actual shipping to hit you up with any additional fees to make up for the under charge.
But as others have said, the shipping companies themselves have been sending additional bills to you after delivery based on tariffs for various line items inside the shipment. It's very random and arbitrary, but can be very expensive. I've been hit up with more fees after the fact than my initial shipping costs were from ups this past year.
Sip at least confirms with you before charging you or shipping your order.