Interesting thread.
I collect stamps because they're a form of payment along with coins, banknotes, cheques, bills of exchange, etc.
Once in a while I bid on postcards and postal cards, especially if they have a (modest) historical value. For example, this 1909 postcard sent from Paris, France:

If you google both Curtis for his pioneering flying ventures and Cook for his reaching the North Pole (which was a much more controversial claim compared to the reaching of the South Pole), you can easily spend a couple of hours on this…
Here is a postal card sent from Austria on May 16th 1938:

As the sender explains, both German and Austrian stamps could be used together for a short time. Why? This was sent two months after the annexation of Austria to the Nazi state, so the sender, on a trip to Austria, took the opportunity to send a postal card to a French friend, “retraité des Postes, Télégraphes et Téléphones”. I haven't yet checked when the deadline was until all sendings had to be in Deutschmark stamps. I haven't checked either whether I can trace the Monsieur H. Fromentin to whom the card was sent.
Even without collecting postcards or stamps, these two items are, IMHO, quite interesting. They cost me only a few dollars each on eBay.
₱o$₮ag€ $₮am₱$ a₹€ mo₹€ £€₲i₮ima₮€ a$ a ƒo₹m oƒ ¢u₹₹€nc¥ ₮ha₦ ₮h€ €₦₮i₹€ "¢oi₦" ₱₹odu¢₮io₦ oƒ ₦au₹u o₹ ₦iu€. ••• £€$ ₮im฿₹€$-₱o$₮€ $o₦₮ ₱£u$ £é₲i₮im€$ €₦ ₮a₦t qu'o฿j€₮$ mo₦é₮ai₹€$ qu€ £a ₱₹odu¢₮io₦ €₦₮iè₹€ d€ «mo₦₦ai€$» d€ ₦au₹u ou d€ ₦iu€.