1) Postage - do not cheat/defraud the postal authorities by using insufficient or invalid postage. A postage stamp can only be used once of sending an item of mail. If it has been postally used but remained unmarked, it is still not valid for postage.
2) Seeing photos of snail mail on social media can be good for inspiration, discovering new ideas, and show that letter writing is not dead. Please, please do hide/obscure/blur the other person's address. UK postcodes are very specific and with the house number, it is just revealing as leaving the rest of the address. 10 / SW1A 2AA is the minimum needed for one iconic London address with a black front door.
3) When writing the date, use the name or accepted abbreviation for it, rather than the number. Date conventions differ. 1st February is not the same as Jan. 2nd. 01/02.
4) It does not matter if the letter cannot be finished in one sitting. I don't worry about pausing letterwriting, even after a short time. I do hope to have finished the sentence I am writing if I have to pause. I have started a letter while writing for a friend on a night out. I am usually able to finish a sentence/paragraph when she turns up.
5) The writing medium does not really matter. Paper meant to be written on, be it lined or blank or dotty or wavy, or with punched holes down the side and used by students to take lecture notes on (a school friend penpal wrote to me during her lessons), or reporter's notebook with pages on a spiral, or even actual letter writing paper.
It doesn't matter if the letter is written with a fountain pen, dip pen, or ballpoint. I admit I find it easier to use a fountain pen as the ink makes the words flow across the page.
The words matter!
6) Of course I want to hear from you sooner rather than later. However, response time doesn't bother me. I like to reply to letters between a fortnight and month after receipt, having slowed down from within a fortnight. Life happens. I know some penpals take a month or more to reply, that is fine. Penpalling is supposed to be a hobby and therefore an enjoyable pastime. If pressurised, it can feel like a chore and you could get burnout. InCoWriMos/LetterMo/Write_on, the February or April letter writing projects, can overwhelm people and drown them in correspondence.
7) If you do not keep the letters/envelopes, consider saving the used stamps for charities (as they can raise money from sales to dealers/stamp collectors). There are plenty of charities in the UK raise some funds this way.
8) You can't be penfriends with everybody. Personalities can clash, beliefs can collide, or you just don't click.
What do you do when you find you cannot write a reply to someone? Is it easier to not say anything? Or do you write to tell them (via snailmail or even via electronic messaging)? I haven't had to make this decision for a while, but I find myself in the category of not saying anything. It may be because I do not feel like writing to them at the time but maybe in the future I will be able to. There is hope. Life happens, stuff happens. Time. Patience.
I think that is it for my rules/guidelines.