You have remarkably high DSRs for the volume you sell so why on earth are you worrying about one buyer who doesn't understand how the postage time works and thinks it applies to the delivery time, not the time you take to post.

DSRs are a comparative tool so that buyer, and others,are purely rating from the their viewpoint so of course there are going to be cases where a rating is unfair, there is also the mobile factor, when people are using mobile phones and small tablets to access the internet where it is very easy for them to 'press' the wrong 'button'

It doesn't matter how much proof you have that the item was sent, ebay don't have access to that info and they are certainly not going to employ staff to verify copies of postage receipts sent to them just so a seller can get a DSR rating changed. Considering the number of global sales and the proportion of buyers who mark stars down either by accident or because they are peeved off at a particular seller or ebay in general they would have to employ all the adults in a small country, do you really want to pay the extra fees that would cost?

With your total feedback you would need to get a couple more low ratings before it had any adverse effect on your account so why not concentrate on continuing to offer what is obviously a great buying experience for your customers and forget about your DSRs.