If you cancel or refund on anything make sure you dont do it in a way to cause you a "can't supply" defect..ie it must be arranged to be by buyer request.

Always best to keep things as simple as possible and not confuse the buyer, or Ebay.

If you get a request about issues like this from buyers before they purchase you can make a reduced cost offer reply via return message.

Unfortunately dificulties with regards to combined discounts is one of the hurdles to "freepost". Ebay cant resolve this until they admit it is post included rather than non existant post.

What buyers dont want to hear is that you have bumped your price up to include postage. Even if they think it they dont want you to come out and say it.

Has it actually made any differences to your best match search standings? Its not the dominant factor it once was.