You could use either of two approaches:
1. Filter the mailmerge (via Mailings>Edit Recipient List), so that only the selected record is processed; or
2. Use a SKIPIF field with a prompt for the user to input the record to process. Such a field might be coded as:
{SKIPIF{FILLIN "Purchase order # to output." \o }<>{MERGEFIELD Purchase_Order}}
where 'Purchase_Order' is the name of the field holding the Purchase_Order numbers.

Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]