To me, designing the sound effects has always felt like a natural extension of the drawings. Computer fonts are brilliant of course, (particularly those by Comicraft) and I've used those too a few times, but I prefer to create a specific sound effect that suits the moment. I'm sure there are professional letterers (or 'balloonists' as DC Thomson call them) who could do a slicker job, but I prefer to do it myself where possible. I don't receive an extra fee for it but, as I said, I consider it part of the drawing. (Dialogue lettering is different of course. I'm paid extra for that when I do it, although these days it's mostly done by the in-house designers.)
Anyway, these are three panels from the latest Team Toxic two-pager, The Land of Living Farts, in the current issue of Toxic available right now!
http://www.toxicmag.co.uk/