Like thinking he ordered X amount but it was actually Y amount, or placing an order and it taking so long you forgot about it so you place it again, or ordering the wrong product code and failing to verify it, or skimming over the order and missing that the salesman added extras for whatever reason and signing it anyway, or…

If he didn’t remember placing the order, or believed that the order was modified, then at worst he’s mistaken, not lying.

Thats not correct. When placing these orders a sales person will put a sku code or a in house reference number on the quote document that doesnt actually mean anything to a normal person and agree to do work that the implementation team cant or wont do (this should sound familiar to people here). Ive seen this done personally.

They also dont charge all up front and might just want a deposit. So they might not realize the cost is higher. Its very easy for a immoral salesperson to try and get a customer to unwilling pay for something. And they have incentives to do so.

The article says it was over the phone. Last month I went to a shop to order a specific size shower which would be delivered next day. The employee took my order, I paid and left. While driving home I got an email with my order, but I didn't check it.

The next day I come to get my delivery and realise it is not the correct size, I check my email and indeed the size is not correct. The shop made no issue and ordered a new one but I lost 4 days.

So who knows?

You seem to be implicitly ruling out mental illness. It's absolutely possible that he earnestly believes this without it being true.