I had about a dozen large letters to send today so I tried this method out on my local PO agency.  The postmistress was more than happy to oblige, even when I said I thought it might be an imposition on her to have to go through this process.  She told me she has other Ebay sellers who photograph all their parcels at the counter.  She happily printed labels for each letter, suspended the sale and set them aside at an area of the counter where I could photograph them while she went on with serving the next customer and then when I was done, came back to me and dealt with the payment.  I said I thought I might not bother unless it becomes an issue for me, she said I would be better off doing it this way rather than waiting for a problem.

I was pleasantly surprised with her reaction (probably shouldn't have been, after all that is why I like to use the little PO even though it is further away, you can't beat the personalised service).  I wouldn't like to suggest such a thing at the big PO though, if you hand them a letter with a stamp on it they tell you to take it oustside and put in the slot in the wall!