Hang on...
Z1M9AN should give you 15% off if you spend over £60 (so no use if you're only buying little bits - then again, "little bits" at Letterbox can tot up quite quickly!) Valid to 24 Dec.
Incidentally, depending what you're getting, it can be well worth shopping around when it comes to Letterbox - you can often find stuff much cheaper elsewhere. I bought some fun fishy bathmats from Tesco for £3 - they were £10 
in the Letterbox catalogue!