That's as simple as a formula field coded as:
{={MERGEFIELD Total}-{MERGEFIELD TotalTax} \# ",0.00;(,0.00)"}
or:
{={MERGEFIELD Total}+{MERGEFIELD TotalTax} \# ",0.00;(,0.00)"}
depending on whether TotalTax is a +ve or -ve value.
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 or, if you’re using a laptop, you might need to use Ctrl-Fn-F9); you can't simply type them (which is what you've done in some cases in your document) or copy & paste them from this message. The spaces represented in the field constructions are all required.
You can probably also lose all the \* MERGEFORMAT switches in your mailmerge main document - they're unlikely to be contributing anything other than clutter.
Cheers
Paul Edstein
[MS MVP - Word]