We have a daughter at school in Newbury who telephones home most days, which enables us to keep an exact record of the arrival of items we post to her.
Post people seem to think that postcards must be frivolous - they cannot possibly contain any urgent message. So postcards sink to the bottom of the pile and always take two or three days. Anything that can easily be identified as a greetings card seems to be automatically sifted out of the first-class system. I have overcome the problem by disguising every card in a plain white business-like envelope - a ruse that is almost always successful.