If you are selling greeting cards that go via letter post you can use this but you must buy your letter post via Amazon.

If you are selling greeting cards that go as large letter you must upload a tracking number or a delivery confirmation number when you confirm delivery. Alternatively you can buy postage through Amazon and you don't need to upload any confirmation.

You don't need to use a fully tracked service. 2nd Class Royal Mail or Royal Mail 48 post is fine as long as you stick with the above rules.