The Worldwide up to 40 grams stamps can now be used on packages up to 100 grams in weight.
These self-adhesive die-cut stamps are 100% valid for postal however the Royal Mail issued them with the backing card flush to the stamp perforations which makes the stamps very awkward to remove from their backing card. In fact you need have very good eyesight and very good nails to stand a chance. Hence why we are selling them off at a large discount to the face value. They could also be stuck on using a glue stick.
We do occasionally use these stamps ourselves on European packages up to 100 grams and rather than trying to remove the card back we just use a glue stick on the card which does the job fine. All of these stamps originate from Royal Mail definitive presentation packs and as such are 100% genuine and valid for postal use.
Although it only says up to 40 grams on the stamp the new Royal mail rates from 30th March 2015 means these stamps now cover you for letters up to 100 grams.