A family of Microsoft word processing software products for creating web, email, and print documents.
Does the issue happen only while printing or while performing the mail merge as well?
Do the postcards show up fine in Word (before printing)?
If yes, you may try printing to XPS Document Writer and check if that helps.
http://windows.microsoft.com/en-US/windows7/Print-to-the-Microsoft-XPS-Document-Writer
If it prints fine to XPS, you may have to update the printer drivers or contact the printer manufacturer.
If you’re not able to perform the mail merge correctly, you may perform a mail merge using an Excel file/Outlook or create a new mail merge list and check if that helps. Refer the links given below for more information: