It's not only fabric shopping which is like that, it's all mail order. Very often the "cheaper" product ends up more expensive once postage/shipping/delivery is included.
£6-99 probably reflects that they send by Royal Mail first class, maybe tracked as well. It would be nicer though if they gave the option of 1st or 2nd class.
I send quite a few small parcels (up to 2Kg). 1st class is £5-50, 2nd class is £2-90 which is quite a dofference. Our localish newsagent is also an agent for MyHermes and I can send a parcel from there for £4 up to 5Kg. Quite a lot cheaper but a lot of people prefer Royal mail.