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What do you mean by "all of those stamps"? It depends on the weight of your envelope.twilighter wrote: so i need ALL of those stamps just to get 1 letter back!?
If your return envelope weighs 20 to 40 grams it'll cost £1.35. If it's between 40 and 60 grams it'll be £1.82.
Since 1st Class stamps are worth £0.39 you'd need 4 for 20 to 40 grams, 5 for a letter weighing 40 to 60 grams. If you only have 2nd Class (or a mixture) the number will be different. It just has to add up to the correct amount.