add text on 1st page of a pdf file

I'm tryin to add a text comment (not a note) to a pdf file. I create a date.ps file with contains the text comment : %! /Arial findfont 30 scalefont setfont newpath 10 720 moveto (PAID on 5.1.2013) show showpage

and I launch the shell command with $i=name of the pdf file to tag: gs -q -dNOPAUSE -dSAFER -dBATCH -sOutputFile=$RFP/$DOMAINE/$NEWNAME -sDEVICE=pdfwrite -sPAPERSIZE=a4 date.ps $i

This works, but it create a new 1st page empty with the text "PAID on 5.1.2013" alone.

I do not find the trick to overlay the text on the 1st page of the original pdf.

Can you help me pls