I think that not even ReportLab could merge an existing PDF, IIRC
there are other tools to open and possibly modifiy files.
If you can make your design and fill it, then you can use PyFPDF
(there is an Invoice example).
Also, sometimes I put an image as background (+-300dpi) and then I
only complete the text fields.
YMMV
Mariano Reingart
http://www.sistemasagiles.com.ar
http://reingart.blogspot.com