Install the plugin Shortcode Exec PHP
Create a Paypal button and place the code in a php file in the plugin directory shortcode-exec-php
Go to tools->Shortcode Exec Php
Create a new php shortcode, with the code being…. include ‘paypal-button-code.php’;
Name the shortcode, save it, and include it in the wordpress page
=====
To create a ‘Pay Now’ button that allows the client to enter the amount first, see:
https://www.x.com/node/2148