hey there retailer!
Life gets busy. Post boxes aren't at the end of every street anymore. So let me take care of it for you.
Pick your card, add your greeting, job done.
You can even pick whether you want it handwritten or printed in a right fancy font - and everything is sent Royal Mail First Class (with an actual stamp so it looks like a proper letter and stuff) so you know it'll be there when you want it to be. Admit it, it just easier...I won't tell.