But you can often get such small items in Pound shops for ehh.. £1.
The price they charge in DIY sheds is what they think is your bother level, i.e. time and petrol for finding alternatives; hence £2.99 is often the minimum ticket price you will see on anything. Sure enough, it doesn't really matter if you need only one packet of screws, but if you need ten packets, the difference begins to show.