£18 a bag is not going to gain much interest.
It’s £20 delivered from Huel (less if buying in bulk)
It would cost more than £2 to go and get it even if you were local to Cambridge.
So there’s no benefit to anyone to buy it at that price. It would actually be cheaper buying direct, especially with the bulk discount you get from Huel, plus most people make use of the student discount or refer a friend discount.
If you really want to sell it fast, lower the price.
Or if you need the money, try eBay: you could ask full price (or even make a profit) as lots of people want to buy single bags on there. But then you’d have the hassle of posting them all…