The collection slip must indicate where you can go and collect the parcel... pop this into google and find their contact details. Or give me the name and I'll search it for you - the branch locator is working just fine for a change. Ask to speak to the manager and get him to explain to you how you would go about getting the package re-evaluated by customs.
Did you just order the Pi, or some additions?
Easier just to go there instead of trying to explain/complain via telephone.
I ordered the RaspberryPi and transformer and SDCARD+PiOs.