Shipping costs weirdness

[judeabb]

I've got a shopping page here: http://www.folkagainstfascism.com/prestashop/ and have also calculated Fees by carrier, geographical zone, and ranges.
I have also disabled the tax.
In the Small Things category, one particular item - the FAF pen - which costs £2 the shipping comes up as £5. I've tried deleting the item and adding it in again but it still happens. If I combine it in the basket with another cheap item - stickers for example, the shipping cost goes down!

What is happening?
What further information might you need to help me sort this one out?

Thanks in advance.

Jude x