I've just sent away for a digital tachograph card which, like the passport form, has an adhesive panel for sticking the proper sized photo to the form. I had the photos done at a local shop who advertised passport, licence etc approved prints. I ended up having to trim the beggar to fit. Seems you can't win.
The PO give the impression that they know what they're doing but request something like a proof of posting certificate and you may find, depending on the operative that they either do or no longer do those.
Hope it's sorted soon and you have a good holiday. :)