I started pen palling when I was about 10 and then stopped when I was about 34. I had the same pen pals for many years and at the most I was writing to about 100 people!
A part of pen palling was collecting stickers to put on letters and envelopes so I got involved in sticker exchanges. How a sticker exchange worked was there would be maybe 10 to 20 members from all over the world. When you received the envelope full of sheets of stickers, you would take out what you wanted and replace them and put your initial in the corner. Then you would send the envelope off to the next person on the list (this is back when postage was cheap). When the envelope of stickers came back to you and sheets of yours were still there (that meant than no one wanted your stickers), you had to take them out and replace them.
When I first joined sticker exchanges back in the 1980s, I was still living in Antigonish, NS, and there wasn't much of a selection there so it was a great way to get exposure to so many different stickers from all over the world!
I kept the sheets of stickers protected in photo albums (sorted by style, content, type, etc. ... yes, I was that anal!). Once I stopped using them, all those photo albums took up a lot of space so I took the stickers out of the albums and put them in a canvas bag which has spent the last 20 years or so in my closet.
I took them out today and went through them and sorted them a bit ... there are hundreds and hundreds of sheets!
I've sort of got them by type ... animals, hearts, rainbows, unicorns, funny sayings, famous characters, shiny, scratch 'n sniff (some still smell after all these years!), etc.
Alas, I don't write any letters these days and I'm not sure what I can use them for ... but I can't throw them out.