[sunir]

What is the difference between a funder and a paying customer?

They are both users who are paying for the service.

The difference is that funders have political power over the project beyond the scope of their own use.

Customers are always better and more fair to everyone than funders.

[ROARosen]

A service that's available to the 'public' is called a 'public service' even though it is funded by 'funders'. But a service that's available only to 'customers' is by definition not a 'public' service.