Mailmerges do not work well with mixed data formats. To a large extent, that is because they are designed to work with databases where only one data format is permitted per field - in cases such as this the postal codes would have to be stored in a text field. With your Excel data source, the simplest solution is to ensure the first few records include data in the alpha-numeric format, so the mailmerge process will interpret it as a text field. You should then be able to do the merges with just:
{MERGEFIELD ZIP}.
See the
Mailmerge Data Format Problems topic in the
Mailmerge Tips and Tricks 'sticky' thread at the top of this forum:
https://www.msofficeforums.com/mail-...ps-tricks.html