There is a C# wrapper for Win32 USP api's from the icsharpcode guys here, and I've seen at least one other reference to USB stuff in .Net.
What may be tricky is whether or not the Printer requires OPOS in order to function. I did some work on a POS project in .Net a few years back, and from what I remember OPOS is pretty much the standard for the hardware side of things (And at the time all OPOS APIs utilized COM interop). The devices sometimes also have a documented proprietary communications protocol, in which case you should be able to utilize that to talk to the device using the USB library.
