It's because an 89p cable goes Royal Mail 2nd class, the fluid has to go via a courier (DPD in this case) due to courier restrictions, hence the delivery charge if you're buying it on its own, though it's still free delivery if you buy it with other items that total over £40/50 can't remember the threshold without looking
There seem to be a number of variables involved.
10 litres of Ultra Pure fluid @ £25.00 attracts a delivery charge of £8.70. 11 litres at £27.50 and delivery is free.
1 litre of Mayhem's X1 @ £6.95 attracts a delivery charge of £8.70. 2 litres at £13.90 and delivery is free.