This is what is known as a "Door Opener".
The salesperson (usually working on commission only basis) has no intention of selling you these blinds at the advertised price.
The colours of fabric available will usually be unpleasant e.g. pea green, mustard yellow or bruise purple.
The intention is to get you, the customer, to choose nicer fabrics that will be a LOT more expensive than the initial £99.
If you persist in ordering the "Special Offer", the price will go up and up and up, once you pay for delivery, fitting and paying extra to have the blinds open how you want them!!