Of course the kit may arrive much sooner in this case ...
It did - in the post (First Class), 10 hours after the 00.30 email in which they told me that they had despatched it "today". (Plusnet has been unable to tell me whether that email was late reporting from the previous day's despatch, or early reporting of that day's despatch).
So Plusnet seem to be alarming people needlessly (First Class post is better than a "3-5 Business Day" service, and - at least in my case - they are shipping earlier than they report). That alarmism isn't cost-free: I very nearly exercised the right (which BT, as my previous landline supplier, had informed me about) to abort the whole job until a day or two before upgrade - because the consequences of their end-of-window delivery date were too difficult to accept.
But Plusnet are still despatching too late. If they pulled their finger out, Plusnet could despatch this kit say eight working days ahead (their minimum lead time seems to be 10 working days), saving money by choosing Second Class, and still give their customers less worry.
But from the conversation I had with their agent yesterday, I'm not hopeful of improvement. He was charming and very professional, but he had clearly had more training on condolence skills ("Yes, I can see why that is worrying you") than on skills in listening to customers who might just be telling your organisation something they need to learn from, and then giving the customer the impression that you might just feed this back to someone who could do something to improve things.