I don't think the issue lies so much in tacking on a bit extra to cover the tax as much as being subject to it without knowing. It doesn't seem unreasonable to

I don't think the issue lies so much in tacking on a bit extra to cover the tax as much as being subject to it without knowing. It doesn't seem unreasonable to expect that building an application for another company would be considered a service. They may have done the research and discovered that service providers aren't required to collect sales tax and moved on. The tax code is such that you can't expect to understand every little component of it unless it is your full-time job.