DLA-PIP posting the form

I need to send my pip form off this week. My date is sunday 12th so from what I've read here I need it to get there by Friday 10th. Considering it took 10days to arrive from the date I got a text to say it had been sent out, I have only had just over 2weeks to fill it in. Doesn't seem fair. How can it take 10days! My advice is for people to print off a sample copy of the form and start filling thst in before the form arrives! I did this and am still struggling for time!. I don't want to rock the boat by asking for an extention.
My question is about posting. I know if i need to i can pay £7 or £8 and guarantee next day delivery. But I have just read a comment where someone's form has been signed for at a royal mail delivery site not the address on the envelope. The brown envelope says 2nd class and the address is Free post. Should I put in my own envelope? Or stick a label over the Freepost if I'm paying for it 1st class or next day?