Hi, The chances are once the letters have been through the postal service with other items sitting on top bouncing around in a truck, the cloth will flatten out a bit and the air may get squeezed from the parcel, so should be a bit thinner than when you mailed them. You may still get caught out, but it is fairly unlikely if you have been sensible about the thickness ( 2 cm. ) . It would pay to ask your local post office for one of the thickness guages that they use so that you can check items before mailing in future, just to set your mind at rest. If you ask nicely they should let you have one for free.