IIRC the library is free but if you want to use their other services then it can cost money
I love the Segment.io product and the fact that the library is open source.
However, they don't allow for easily just adding a number of vendors and having that data be sent out to each vendor directly. Rather, they want you to go through Segment.io, then they send it out to each from their back-end service.
I needed something that allowed some level of abstraction for multiple vendors and call one master analytics library that does that for me. There's more that can and will be done with the code base to make adding vendors more modular, while keeping the overall simplicity up.