It seems to me that all you need for this is to apply an IF field at the end of the mailmerge main document test to your 'endorse' field, coded along the lines of:
{IF{MERGEFIELD endorse}<> "" "SlipSheet"}
Where 'SlipSheet' is the actual Slip Sheet content, rather than a separate PDF.

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]