We are often posting out letters in various sizes and also small packages. We have a letter scale to weigh the letters, and a sizing guide as well, both of which were bought on Amazon. Local post offices don’t sell them. We might complain here that if you post a letter, then the Royal Mail has three ways to discover that it is understamped. They can find that it is heavier than anticipated, or bigger than anticipated, or thicker than anticipated. This is contrary to the spirit of Rowland Hill, the postmaster who introduced the penny post in 1840. It is probably the principal reason why not so much is posted these days when any competent business would have exploited the Internet to increase the volume of physical postage in tandem.
When mail arrives at the Sorting Office, one of the first operations is to separate it between small letters (postcard, DL and up to C5 sizes) and large letters (C4, foolscap and gusset envelopes). This is done in a large rotating drum and you can watch examples on YouTube if you wish.
If you post a small letter, where the envelope is DL size or C5 size, and it weighs less than 100 grams, and it is thinner than 5 millimetres (watch this one), then a single standard stamp will do the job. The letter is processed automatically. One machine can read the stamp in ultraviolet light, and another machine can read the address using optical character recognition and print a UV-readable code on the letter. If OCR fails to work, which is about 10% of the time, then a picture of the letter is sent to a human operator who types in the postcode.
If you post a small letter first class, and you catch the evening collection (5 pm in our nearest supermarket post box), then first class mail will be processed and on its way that very night. Second class mail will be left for processing the next day, and presumably the Sorting Office adjusts staffing levels to ensure this. Here there is a real difference between first and second class mail.
If you post a small letter for the morning collection (9 am in our nearest street post box) then again the first class mail will be processed first, but the second class mail could also be processed the same day. The critical operation is getting a human to read 10% of the mail, where first class gets priority. After that everything is automatic and first and second class mail travel together. You might find it pointless to pay extra for first class post in a morning collection. Just think about exactly what you are expecting to happen when you pay extra.
As we say, processing is fully automatic for small letters and the stamp itself is readable in UV and tells the system what class of mail is intended. There is no point in putting a sticker saying “First Class” on the letter. It is best just to know your local post boxes and choose the one with the next collection to do your posting. We call this the “Night Mail” system because something useful is always happening at night if we choose the right box, even if we miss the 5 pm collection, and combining this with first class stamps gives the next best thing to e-mail.
Larger and heavier letters get separate handling. A4-size letters with things like bound accounts in them are called “flats” by the Post Office, presumably because they resemble flat parcels. There are such things as flat-sorting machines, but we can guess by processing times that they are not always used. You should therefore assume that large letters are processed manually. If you post something large and a bit heavier with several stamps on it, then these can be scanned and added up in UV light, but it might be useful to put a FIRST CLASS sticker on the envelope next to the stamps. These can be bought on Amazon, but we print our own. Ours will say things like FIRST CLASS NIGHT MAIL in winter, which is a cue to ourself to post it in the best local post box for speedy collection, and a cue to the sorter to pick it out and send it on its way.
Like the Post Office, we also use OCR, but to scan bank statements, and this gives us a Unique Selling Proposition. Not every client of ours can have a USP, but we also advocate having a distinctive product. The competition should find it impossible to copy the USP, and practically difficult to copy the distinctive product. The way we use both e-mail and the post is part of our distinctive product and puts us in a better position as business advisers.