I find the best results occur when using wildcards. in your examples, *shipments* would include anywhere the string “shipments” occurs. this would include “shipments” and “shipments!”.
I have noted that sometimes search results work without wildcards, but they _ always _ work with them.
As a follow-up, I have been told by eM Client that searches start at the beginning of a word and looks for the entire word. While I don’t always find this to be the case, assuming this is always safe.
That is why I generally use the header option, because you don’t need wildcards. Subject: ship , will search for *ship* in the subject. 
Where did you find the information about searching the header? I’ve looked through the Help details and I found nothing.
Not everything is documented, but I am sure I saw this somewhere.
When using the header option, you specify which field to search in, and what to search for. It is in the form of Field: word
This example is an OR argument, so the search will return any messages where CB is in the sender’s name or address, or where forum is in the subject.