Why does it take so long to get hard drives from Amazon stores?

I have a NAS and need hard disk drives that are compatible for it. I bought two from one store that said they had them in stock, but after a week they emailed to say they hadn’t got them and couldn’t get them. They were only a common brand, nothing special. They then offered me two different drives not on my compatibility list but I had to phone them.

In fairness when I cancelled they dealt with the cancellation straight away. I won’t name, them but in future I’ll never buy from them or recommend them.

The second store I used took several days before saying they had been dispatched. Then after a few more days sent another ‘dispatched’ note saying they were to be delivered 1st class by Royal Mail ‘probably the next day’. But they followed this up by saying it can take up to 5 days! What tosh. Still waiting.

Surely there must be an Amazon store that can deliver hard disk drives overnight? I know of one company, Dabs, that does, so maybe I should have used them.

Update

The hard drives just arrived by Fedex! Weird…