Many moons ago, Nominet, the UK domain registrar, confirmed the registration or renewal of your domain with a physical letter. I remember being very pleased to receive mine:
I have a hazy memory of them sending me a certificate on first registration, but I can’t find it anywhere so I could be making that up.
They must have been sending out thousands of letters a day, and I assume it was an admin nightmare because it took them three months to send me this letter. Shortly after this they moved to an online system, and this was the last one that I ever received.