David Spain scribbled something like ...

As a customer, I would give you the dimensions and the cost to 'rent'
space on the bus. You would have to design your payload to fit within
the bus and be compatible with the bus ejection mechanism, but that is
about it.

The February 2011 Scientific American has an article on the CubeSat
program, which already fits that description: "Citizen Satellites", pp 44
and following. The SciAm web site will have the opening paragraph of the
article available for free and the rest behind a paywall, but most
libraries will have hardcopy.

/dps