A minor point, but if the federal government wanted to run an ISP (inside or outside of the USPS), they don't need anything new in the constitution to do it.
If the Interstate Commerce Clause can be used as justification to create the departments of Agriculture, Education, Energy, Health/Human Services, Housing/Urban Development, Labor, and Transportation  as well as the FBI and just about everything else, I can see no reason why it couldn't be used to create a national ISP.
As to whether it would be a good idea, I don't have a strong opinion.
1. Some might argue the Department of Transporation is justified under article I, section 8, paragraph 7 (not paragraph 3), though I don't think either theory has ever been tested