Today I got here a label printer to test: The DYMO LabelWriter 450 Turbo.
It is connected using USB and comes with standard drivers for Mac OS X and WIndows. This way you can use it in all REALbasic applications with the normal printing commands. Works very nice here for the application we needed.
Last year I had a different one which had only his own print commands so I wrote the CUPS plugin part to control it on Mac OS X. The only downside is that it supports 50 paper formats and you need to choose yours from the paper format list. Maybe an auto detection would be good.