Yeah same thing happened to me the one time I ordered from GameStop in the US. The worst part? The pre-order item I ordered from them for wasn't even included because they messed up my order.Had 3 orders with them this year, the last one being Dragon's Crown on Vita and PS3 in one package both with preorder artbooks so was quite a big weighty package and they still marked down the value to get in under customs.
I spoke to a UPS guy a earlier this year and he said that stuff going through the postal service at either end (ie Royal Mail) very really get flagged unless it's a big item but pretty much any courier end consignment will get checked out. UPS even includes clearance fees as part of their service as they know I their stuff will get looked at. GameStop for examples did me over with my US copy of The Last of Us and ended up costing me £35 in taxes as well as the $100 I had paid them for inclusive cleared shipping.
Media such as games, DVDs, blurays don't carry any import tax (classed as category 9) but anything over the value of £50 is subject to be checked (sometimes as low as £15) and then everything is due 20% import VAT if it gets picked up.
I don't mind being charged customs if they didn't charge the ludicrous fees for paying them. Most of the time the fee's are more than the VAT itself. Like before I've had £5 VAT and £12 Fee. >.>