Thanks for the feedback guys.
Yeah, like I said, if it were up to me, I'd sell for £6-£7. I only got ten, with a 5% discount so they cost me £38. Red Brick are confident they can sell for £10, although if they do really well there I will be charged a commission. Not that I mind that, as they give me support that goes far beyond merely financial.
I know of the likes of etsy but never looked into it in depth. I'm a bit timid about doing this sort of thing and I usually need somebody to help me do it. A print on demand site like Zazzle might be most convenient, as I can upload multiple pieces of art and see which sell best. I think a lot of my random doodles would sell much better on mugs than it does on postcards though.
And yes I would much prefer to put my details on the bottom of the mug, but the site I used - tshirt studio - didn't give me that option, unfortunately. There's a lot of trial and error going on here for me, but I've clearly underestimated the initial appeal because they've so far generated a lot of interest. Yes, it's from people who know me and are happy to support my work, but it's a vital confidence builder.
I'd be happy to have a mates rate of, say, £7 for one and two for £10. At this moment, at least when it comes to the Red Brick stock, it's finding out what strangers are prepared to pay. I can always drop the price if they're not selling, and if they still don't sell - well Christmas 2018 will be sorted for a lot of people!