From where? To be on the safe side, take the letter, which could be classed as a large letter or small package, to your local post office. They will weigh the letter and match the postage to the size and dimensions of the letter. Having dealt with postage issues in a British company before I retired, nobody would be pleased to receive a letter they then had to pay the excess stamp duty! Also, would the letter be going by airmail or the slower seamail?