Pip postal address

We are currently filling in the pip form.
The deadline is 15th July but school are giving me a supporting letter next Tuesday.
That means I'm cutting it fine with the free postage label.
If I pay to post it do I need to write a new address label or can I still use the freepost label on the back of the form?

That's great thank you. I'll do that, although I got a letter today saying they've given a 2 week extension!
I'd still like to post it tracked though