Royal mail..... or post'n'hope as I call it. It can be so variable, one day it runs like greased lightning or at the other extreme , not at all.
Then there are the letters which arrive on time but the contents are missing, I've had that happen too. I sent a small (tiny) component in an envelope across the county only to receive a phone call from my friend stating that I had put the small polythene bag inside but had not inserted the component. On examination, we found a tiny quarter inch cut in the corner of the envelope and a corresponding one in the polythene bag..... it appears a postie had nicked the contents and guess what, Royal Mail couldn't care less.
This also happens when you put cash in the post, birthday cards and the like are fair game. The technique used is the same, a small slit in the bottom corner, insert a wetted pencil, roll it round so it grabs the note and romove the pencil with the note neatly wrapped around it. The recipient is unaware that a note ever existed and by the time it's realised it should have been in there, the envelope has been discarded..... not that it would make any difference since you will get short shrift from RM.
My advice is never ever send anything of any real value with RM if you can avoid it.
Carriers are a different matter, I find I can usually sleep at nights when I use them although there have been one or two occasions when something got damaged, but it's extremely rare . All my expensive stuff goes with them and it goes all over the world. I can get a boat to America or Europe quicker than RM can get a small package across the UK sometimes.
