It would be nice if they never lost your form, or even your whole file. But they do. So keep copies of everything you’ve written, and send copies of everything someone else had written, just in case. See how not to let this drive you mad in paperwork hints
Often there’s just too much to go into the envelope they give you. Then, use an A4 envelope (I save these) and tape theirs on to the front so you can make use of the postage. You’ll need it weighed, and the postage checked at a post office or here online. You can add stamps to turn second class into first class, and/or to pay the extra for recorded signed for.
I often need to post first class, but I just get the free postal receipt from the post office unless I’m really pushing a deadline. Then, I take my forms to the jobcentre where they will put it in their internal mails and mark it as received on that same day.
They may not give you proof, but you can always photograph yourself (datestamped) with the envelope outside the jobcentre – and it’s not failed for us so far. If you’re too ill to do this, then you’ll need a friend to do it for you.
Keep a note of the address you are sending to on your copy of the form. You’ll need it to send in any evidence that arrives after you had to post.
Always add your name and NI number to anything you are sending them.