PostNet Bar Code

I am trying to fit the Zip+4 Postnet Bar Code, on the Avery 5260 Labels. I am using an Excel database, with Word-2000. When I prepare the mail merge master, I reduce the font to 8, using regular Ariel. However, the postnet bar code for any zip which I have the +4 digits dose not fit the label. I called Avery they could offer no solution other than use the 5263 Shipping label. This label is too large and expensive for our 300 member newsletter. The post office give a Five Cent break for using the Zip+4 Postnet; $15.00 savings.