I don't think you can blame the shipper for the Post Office's scr*wup...

If it is a scr*wup. With FedEx to some extent and especially with UPS, you get only the delivery time you pay for, regardless of the actual transit time. Parcels will sit in UPS's warehouse for up to a week, to be delivered on the scheduled day, if you've only paid for ground shipping. I'm not aware of USPS doing that, but companies tend to copy what works for each other's bottom line.