Charging for installing PostgreSQL is not the same as charging for PostgreSQL.
Bottom line: you charge for services you provide not for software that other people provide.
That's just really not true. There is nothing that prohibits you from selling Postgresql. I mean, it's not a great business model because you can get it for free, but there's nothing that stops you from doing it.
Quoting Adrian Klaver in this thread from about eight hours ago: "You cannot (legitimately) charge the pharmacist for any part PostgresQL."
Actually that's Rob Sargent you're quoting. Adrian took issue with that statement, as do I. While Google isn't finding me anything that says "Yes, you can sell Postgresql," here are a few points:
Point to anything in the license wording that says you can't charge money to distribute Postgresql. You can't.
Even software licensed under the GPL, which is a considerably more restrictive license, can be sold. The free software folks consider the right to sell as one of the freedoms associated with free software. [1]
The Postgresql license page says it is "a liberal Open Source license, similar to the BSD or MIT licenses." [2] The MIT license itself explicitly states that it grants rights to "sell copies of the software."
How do you sell what you don't own?
You can do so because the owners have granted you the right to do so. They were just good enough to not charge you money for it.