I know about the literacy rates of the era, but I just started to wonder about something : What did people who just wanted to write a small thing, or people who were learning how to write, wrote on? Like, wasn't parchment super expansive? I know wax tablets are a thing but were they back then (And were they affordable?)? Did they write on un-dyed pieces of cloth? Did they write on goddamn bark?
Like, let's say I'm a random low-ranking noble who happens to be somehow literate (Or would I just ask the priest/clerk if he could write something for me?) and I want to write a small message to my beloved little sister (Who also happens to be somehow literate, or have someone at hand who could read the message for her) for her birthday (Going on the principle that I don't want to send the message orally and that she lives far away enough for me to not really want to go give it myself) : What would I write it on?
By the way, did random nobles send out messengers/knights/squires/men-at-arms/random-servant-with-a-horse to send trivial messages like this? Or did they just gave the message to a random merchant passing by in hope that it'd reach its destination?
I can't answer all of those questions, but I can shed some light on some of them.
Paper was manufactured in Europe from around 1150 (it was first made in Spain around this date), and so wouldn't be available for the entire period you are describing (see Mark Bland, A Guide to Early Printed Books and Manuscripts (Oxford: Wiley-Blackwell, 2010), p.24. While cheaper than parchment, paper remained an expensive commodity - it wasn't cheap to make, and at least in Britain, there was barely any native paper manufacture and it had to be imported (Bland notes that before 1690, paper was taxed at 5% [p.31]). It was an expensive enough luxury that it wouldn't have been terribly widely accessible. Paper was not considered as prestigious or durable as parchment, and parchment remained the dominant writing material for important documents for some time after paper was introduced.
As for whether paper would be used for inconsequential writing - it is important to note that literacy is not the same as having the ability to write. The skills of reading and writing were taught separately, and writing was a skill only acquired by those with the wealth to stay in education (see Margaret Spufford, Small Books and Pleasant Histories: Popular Fiction and Its Readership in Seventeenth-Century England (Cambridge, Cambridge University Press, 1985) - it's mostly about a later period than you are asking about, but gives some good insights as to how literacy rates and writing rates are different. See also Malcolm Richardson, 'Women Commerce and Writing in Late Medieval England' in The Late Medieval Epistle ed. by Carol Poster, Richard Utz, Richard J. Utz pp. 123-146 (p.134). You're looking at a reasonably small number of people with the education and means to treat writing, and writing materials, as inconsequential.
Some letters at least, were written on paper (see here for a digital resource on some medieval letters). I imagine that a letter sent to someone very important may still have been written on parchment.
As for sending letters - where literacy rates were still low, and writing quite a specialised task, you probably would get a scribe to write your letter. When it came to sending it, there was no formal postal service, so yes, you would send letters either with a trusted servant, a courier, or a merchant.