Okay, I did some experiments. The news is not good.
1. Using Adobe Acrobat 8 Standard, I set password protection on the PDF I created. The only things allowed are printing and accessibility. I uploaded it. I set URL code to http://www.e-junkie.com/ecom/oplug.php?ej_stamp_pdf=true&pdf_allow_copy=n&pdf_allow_print=y&pdf_pwd=password9. I used a discount code to buy the PDF for $0.
1.a. When clicked the download link and opened the file in Adobe Reader 9, the PDF had the stamp on it. Security on the PDF was set correctly BUT Filling in Forms was also set to ALLOWED (contrary to how I set security when I created the PDF).
1.b. I then opened the PDF in Acrobat 8 Standard (the full app). Commenting, signing, creation of template pages, filling of form fields were all ALLOWED.
2. I modified the password in the PDF to password$9 (adding a $ sign) and uploaded it again. I modified the URL string to include the new password with the $. Bought and downloaded the PDF.
2.a. The purchased PDF does not have the stamp on it. It seems the $ sign is not supported in the URL code. Security is set correctly when opened in Adobe Reader.
2.b. Security is correctly when PDF is opened in Adobe Acrobat 8 Standard. Only printing and accessibility are allowed, as I set them when I created the PDF.
3. Suspecting something odd is going on, I re-did step 1. above. I set the password back to password9 -- no $ sign. Same security settings. Uploaded the new file.
3.a. Purchased and downloaded the file the same way. Opened it in Adobe Reader 9. It has no stamp on it. Security properties are as I set them.
3.b. No difference when I opened the downloaded PDF in Acrobat 8 Standard.
My conclusion is that both the stamping feature and the security features work inconsistently.
Unless something goofy is going on on my end that I don't know about, more work needs to be done before this stamping feature will work properly and consistently. I urge e-junkie's techs to keep at it -- this is a VERY nice feature that would mean a lot. Thanks for taking on the task.