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Good news, after my message saying I was not happy to pay 3 times the postage he has sent a new invoice with all 3 for £2.85 post, I have paid and thanked him.
Chris, in the case of your cd seller, he could just put 3 smaller envelopes in a large envelope or a box. Royal Mail will send any 'small package' up to 2Kg weight for £2.85 UK 2nd class postage. A small package can be the size of a shoe box! I also find that for small packages rather than letters there is no difference in delivery time between 1st and 2nd class post.