Just discovered this today - might be helpful to know. Royal Mail do not deliver the hard copies of forms. Apparently they open them and send the contents electronically to DWP. God knows how long this process takes or at what point DWP logs them as received!
Paid to send my PIP review form 1st class with proof of postage as only managed to complete it by 18/9 and was due 20/9. Received a PIP cancellation letter 3/10 and when I rang DWP they said it was received on 30/09! Thankfully they did reinstate PIP for now.
