I quite enjoy Redbubble and Teepublic myself. Redbubble has great customer service and decent products AND you can set your own profit margins. Teepublic has frequent sales including $14 tshirts (USD) and whatnot. I bring in more income from Teepublic on the regular than Redbubble actually, and i’ve been on redbubble for years and years, where as i’ve been on teepublic for less than 1 year i believe. Redbubble doesn’t tell you about sales until pretty much the day of, which is really annoying for planning your marketing but Redbubble is a highly trusted source. Teepublic tells you at least a week in advance which is GREAT, AND they give you templates to use for ads and stuff if you don’t want to make something from scratch every time. (sometimes even animated ads too. Their designs teams are pretty awesome!)
I have tried other places like Society6 but S6 doesn’t allow you to set your own profit margins outside of art prints, their upload system is WAY too time consuming (you have to upload a different sized item for each thing, while Redbubble allows you to upload one big thing and activate the things its big enough for. Also easier to change sizes and placement on Redbubble than S6.) I think S6 also really caters to specific art styles. I never made a single sale but my own purchases in the 3 years i tried to be active there. So i’m not sure really. S6 also has terrible shipping times. (they are never close to what they say, even when they factor in cross border shipping. And i don’t mean by days. I mean a 2 week item takes 5+ weeks every time i’ve ever ordered. Even shipments from USA centers for Redbubble and Teepublic, to Canada, usually take ~2 weeks.)
I also agree that Cafepress and Zazzle are the pits. They used to be good but they are a mess now.
However, I actually encourage setting up more than one shop even if it’s got the same items. There are different audiences for each print on demand place and people do have preferred places to buy from. I find that Teepublic and Redbubble suits me well. Both are decently easy to upload to so the process is fairly quick, and they both have vastly different sales. While Teepublic has much less item choices and a few restrictions (like no double sided shirt prints and less options over all) i quite like their products and the sales they do are REALLY attractive. (and while the profit margins ARENT customizable, they are LARGER margins than most other places by default, which makes it very worth it for me.)