After reading that, I really want to know what company it is- so I can remember to avoid them and others can too. EDIT: I see you have mentioned them. They won't be getting my custom.
No matter what the price of the extra item was, you payed for it, and it took you over the £25 for free delivery. Many places I have dealt with in the past have actually said to me if I add something extra, I will get free delivery. They seem to care about customers, and those are the types of places I want to support.
Even local takeaways do the same thing when you phone them. If you need to spend an extra say £1 for the delivery, they tell you, even if the delivery costs more than the extra £1 item would.
I hope you get a refund through paypal ASAP, and steer clear from these people in the future.
EDIT: I forget the name of the place I got my efusion from as I think they are no longer trading, but they was fantastic.
I had a Wismec DNA200 on preorder when there was the screen shortage and the RX200 came out, and after a week their shipment turned up- of RX200's.
The person was very apologetic, and offered to refund me with extra on top for having my money for some time. I ended up going for the Efusion, and he sent me a free £25 replacement Li-Po, a bottle of premium juice, and a £10 drip tip.
Also, he kept in regular contact, going so far as to phone me with updates on what was happening. That was a fantastic company, and I am sad that I forget its name.