You know not whereof you speak, well, not fully, anyway.
Papers can be served by post, yes, and any form of post will do, even not-signed-for types like plain old first class.
But they can also be served in person, and for some classes of paper this is probably a better way. You, as originator of the papers, do not have to do it yourself, and it is most common to employ a private detective to do it, especially if, as in my case, the servee is 200 miles away.
In extreme cases, they can even be, in the words of an official to whom I spoke, be "nailed to her door" (in my case, the defendant was female). I protested that my name wasn't Martin Luther or anything, which raised a chuckle, but apparently it is legally valid in England, provided that you swear before a court official that you have done it.