It's understandable but I wouldn't call people "spoiled". If UPS or any other shipper posts the term "guaranteed delivery", then it should be exactly that, regardless of the situation. Hence, do not guarantee a delivery, rather post it as "estimated delivery" if that better fits the current situation.
I am sort of experiencing this again with UPS regarding the delivery of my new fork. One line says "Guaranteed by May 13". Another line says "Estimated delivery May 14 or May 15".
Tracking's last update was a departure scan from Harrisburg, PA at 4:48am today. I probably shouldn't hold my breath for delivery tomorrow, as "Guaranteed".