Don't design with vias under 0.3mm and trace widths less than 6 mil.A common recommendation is JLCPCB, with the caveat that you will pay more for DHL shipping than the boards themselves on most orders. They offer 2 layers, 100mm by 100mm for $2 and you get 5 copies.
Another option is OSHpark, which is apparently in the USA. More expensive and faster perhaps. They have different grades, but "prototype" is $5 per square inch and you get 3 copies of your board.
I ask my friend Andrew, who has been a long time user of KiCAD and he saysWe keep the shops American, but they change names/owners a lot. PCB Way is of course awesome, but you're feeding the monster if you go there. There are US shops like PCB way, but a lot of them are fly-by-night. I look for places that do batch jobs. Turn around might be as bad a month.
Some of them just outsource to PCB Way. Ancient Chinese secret!!!!
Tom's Electronics pages / tom@mmto.org