My first example--called "10" like BASIC lines were numbered--has all of the setup details.
Example 3 is about server validation.
My first goal for this example was to first to have only the one "PayPal" button. This doc page showed the relevant example.
My second goal was to resize, which took both standard CSS and customized PayPal stuff, as documented here.
Goal 3: change the price. The result is in this source code. That simply took grinding away.