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. Also, many of the stamps offered for sale on ebay are NOT valid for postage, and some may even be forgeries (and could this even fund organised crime?).
Royal Mail does have a reasonably accessible postage price finder on their website. I expect other postal authorities do too... Facebook groups have so much misinformation.
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. Please do NOT share someone's address without their permission. I know I have on occasion, not realised an address was visible in a post.... and when I do realise, I remove the post. I feel utterly mortified I have not been diligent. Addresses can be out there in the public domain, but that doesn't give you good grounds to share them.
As for sharing the contents of letters, the writer of the letter has more leeway. Incoming letters, the juicy contents shouldn't really be shared without permission, but the odd word or phrase... e.g. the date, greetings/salutations... maybe could be - you want to show off someone else's gorgeous (or not so neat) handwriting, or the ink they've used, ... It is a judgement call. A few years ago, I planned to do a blogpost on handwriting (not calligraphy), on the different styles of handwriting the letters arrived in. I did start preparations, so have photographed some common words (Dear, the date, Thank you for your letter, United Kingdom), but have yet to complete a post on it.
Today's date is 11/02. What? 2nd of November? Much better to avoid confusion and write out the month.
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 letters while writing for a friend on a night out (oh how I miss this). I am usually able to finish a sentence/paragraph when she turns up.
It does not matter if you do not reply to letters in order of receipt. The content of some letters may demand a speedier reply than usual, or you need to delay a reply because someone is moving address and wants to get settled. People have their own rhythm. Mine tends to be a reply within a month of receipt. Occasional letters are fine too. Penfriendship doesn't have a set timetable.
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.
Embellishments, including stickers, washi tape and whatnot are not vital for a good penpal letter. I do use them myself though, because embellishing brings me joy. I cannot draw so some of the stickers can illustrate for me instead.
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! The content matters.
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.
Letter travelling times may be slower than usual... One letter from France to the UK took 2 days in January, but a letter from the same person posted in June, only arrived this week! I'm not one for asking, "Did my letter arrive?" after only a few weeks/months. I know life/events happen making snail mail not a priority. However, sometimes a short missive, either postal or electronic, can make a big difference.
You can't be penfriends with everybody. Personalities can clash, beliefs can collide, or you just don't click. Penfriendship, like friendship, can wane... How many of you are in close contact with your best friends from primary or secondary school, or the neighbourhood you grew up in?
With the February letter writing projects, where you may receive letters from strangers, you might not feel able to reply to some of the letters. I know years ago before the Internet was more widely accessible, you could place an advert in a newspaper or magazine. One letter I received was from someone in prison (death row), and I didn't feel able to reply to it (I was a youngish adult), so did not reply. Other letters you might want to reply with a thanks (either via electronic message of some sort, or via the snail mail) even though you may not want the correspondence to continue, and that is fine too. Though sometimes perhaps, you don't know if you want to continue or otherwise. Perhaps there's hope. Maybe something happens, a word is said or a place is mentioned, reminding you of a letter you received a few months ago and spurs you on to reply. Perhaps an occasional correspondence. There is hope. Life happens, stuff happens. Time. Patience.