Royal Mail aren't really fussy with Christmas temps. I got an 'interview' (which really was checking you were legal and giving you the job), with no issue so you're pretty much guaranteed to get it. I didn't take them up on it or go to the 'interview' because I got a job with Boots instead, but it seems like a very easy way to get some money earned (and if you do nightshift, especially on a Sunday, you can make a decent amount!)