He draws 3 mostly stick-figure cartoons a week, and was smart enough to set up his web store so it takes care of itself as much as possible.
Also, since xkcd is his main gig, technically he’s doing this on work time, not spare time.
That’s not all that impressive. All you need is Google autocomplete and a graphics program.
On the other hand, if he were to do the same thing and make it into a functioning QR code… That would be impressive.
Enough free time to make whole comics for awful puns. I love this one: