The Internet was "commercialized", in the sense of being opened up to commercial entities and anyone else who was willing to pay the freight, in the early 1980s. Of course, there weren't many such people then, since most people had never heard of the Internet, and the societal infrastructure to make it worthwhile for ordinary folks to use it just wasn't there. The government, by which I mean just about everybody concerned in running the ARPANET and designing and building the nascent Internet, knew it would have to be a self-sustaining commercial enterprise in order to succeed, and that the government just doesn't do that sort of thing, and terrible things happen when it tries. So, a very delicate balancing act was necessary.
The CSNet Project, aka the Computer Science Research Network, was an NSF-funded project run through NCAR to see if it was possible to sell access to a TCP/IP-based Internet, and turn a profit doing it. There were four institutions involved: The University of Wisconsin, The University of Utah, The RAND Corporation, and BBN. The Network Operations Center was at BBN. By special dispensation, CSNet and its users were legally allowed to use the ARPANET as a backbone transit net (had to, it was the only national TCP/IP net around at the time). So, commercial traffic on the ARPANET was allowed, starting in the early 1980s...as long as the traffic came from CSNet. CSNet also came up with its own method of encapsulating TCP/IP packets in X.25 packets, so that people could buy commercial X.25 access (which was available) and run TCP/IP over it, communicating with the ARPANET and the rest of the nascent Internet using gateway machines run by CSNet.
CSNet was a success. Not a huge one, but they proved it could be done, and with all the inefficiencies of a government project, too. At that point, commercial entities were willing to stick a toe in the water, and some of CSNet's customers turned around and became Tier 1 providers. NSFNet followed on, CSNet was rolled into BITNet and eventually rolled up entirely.
In 2009, CSNet was awarded the Jon B. Postel Service Award by the Internet Society.
(Full disclosure: I worked on CSNet almost from the beginning.)