Posting PIP form

Hi, just occurred to us today as we posted off the PIP form for my partner Jonathan. The envelope enclosed from DWP, had 2nd class pre-paid stamp. It needs to be back to the DWP BY 18 May 2019, - thats Saturday and they probably aren't in the Office then! So if we wanted to use the 2nd class pre-paid stamp, it would need to be sent Monday 13th to allow 2-3 working days to arrive (Tues, Weds, Thurs) on Friday! That's a whole working week of the time allowed to fill in the form!! Charming. Sent it 1st Class and requiring a signature