I recently worked on a small website projects that required some text changes and a change to a music player.  The project fee was a nominal amount, which I didn't mind especially since the website promotes the gospel of Jesus Christ.  Nevertheless, when the site owner went to make the electronic payment, she actually wanted to pay more than the amount stated on the invoice, but couldn't because it was pre-programmed with the agreed-upon amount.  This experience has sent prompted me to design a Send Payments page. Currently it uses PayPal, but will be updated to accept payments via Google Checkout.