Inconsistent customs charges

Before Xmas I ordered some t-shirts from a couple of different suppliers in the states, two from the suppliers of User Friendly merchendise, Zazzle, and two from mozilla.org. Both packages turned up in the last seven days and I'm very happy with the quality of the gear (I'm wearing my Firefox t-shirt as I type) but I'm a little confused by the fact that the Zazzle order attracted a customs fee of £4.99 and a 'Royal Mail Clearance Fee' of £4 whereas the Mozilla order attracted no fee at all, despite being slightly more valuable.