The dates offered are not guarantees of being fulfilled. They have purposefully built their system to allow you to select imminent dates, with no intention of delivering.

On complaining, they advise that there is a separate Terms and Conditions page that says delivery dates are just a guideline, and can be between 1-3 working days after the date you choose. The invoice you receive gives no such clarification (and of course they could choose to have the system not offer dates that they can't fulfil, but choose not to).

Avoid at all costs.