I’ve dealt with suppliers and shippers for decades, I’m not going to pick sides but reading your post it seems like there were several at fault.
(1) USPS would be my last choice for expedited shipping
(2) Once it was determined the parts were on back order I would have found them through another supplier and once the parts were in hand cancelled the Spruce order. The reason for not canceling until afterwards is you keep your place on the B/O list just in case you can’t find them through someone else.
I also find it odd that the shop has had multiple issues with Spruce while the majority have not.
Generally if ordering parts that I need expedited I do it by Ph not online, the reason is if something shows up as B/O or limited stock their computer screen shows a lot more than you see online, there is generally Alternates that they can substitute on the fly if needed. I have also had suppliers physically go out a pull the part(s) if Qty was limited just to insure the order can be filled.
it is also common for a “Next Day Air” order to have other items added, it’s a flat fee up to a point so adding a few small items for other customers would not change the charges as long as all the items ship from the same warehouse.
COVID has definitely screwed up almost every industry, the biggest problem is it’s too easy for a worker to call in and say they have a fever or might have been exposed and there is little an employer can do, personally I think it’s being used as an excuse by a large percentage of the workforce and is crippling our economy
I know this is of little help to you and is after the fact but in the future if your needing something NOW, phone it in and use UPS or FedEx, rarely do things go wrong this way. Some do not/want to handle ordering items on their own but it is the only way to insure that it did get ordered on time and shipping was correct, this won’t eliminate the shipper dropping the ball but it takes other people out of play... if an order comes in wrong or doesn’t come in on time it was either you/them or sometimes both at fault.
As a side note, Red Label (over night shipping) has a cut of time to have it ship that same day, so if it was late in the day I could CALL in an order and have it shipped from a warehouse on the west coast and have it the next day