I used to send €10 O_o
Mostly because I liked getting all the different stamps to send it with.

It would take quite a while in the end. Print address to envelope, print code, insert money, close envelope, take out all the stamps, calculate the combination to get right value, check the available stock for each stamp, recalculate (no way I am using ones out of stock), take a soft brush, dip in water, wet the stamps, stick them on, make sure they're on well, let dry, take to mailbox, wait up to 10 days for it to arrive and get credited.

You could print a random letter or graphic(s), preferably on heavier paper

I was expecting this at the first half of the sentence: https://27bslash6.com/overdue.html