thanks for the kind words about the PDF-XChange Viewer. We have tried to make using Stamps as easy as possible - this includes a thing we call the 'Stamps Palette' It can be used to create your own stamps from images or PDF files. Take a look at this article, it details how you can create your own collection of stamps. http://www.tracker-software.com/knowledgebase/174-How-do-I-use-the-Stamps-Palette
and more detail on using images specifically can be found here: http://www.tracker-software.com/knowledgebase/62-How-do-I-create-stamps-for-use-in-the-Stamp-Tool
I found myself frequently wanting to put a check mark on non-fillable forms and so added to my collection a series of check marks and crosses and you can do a similar thing for your PAID', 'STILL DUE', 'OUTSTANDING', 'PLEASE PAY ASAP', 'OVERDUE', 'THANK YOU!' etc. Create what you want in any application them either take a screen shot or print to PDF and you have the basis for your new stamps, customized and exactly what you want.