I keep playing with the code and I don't think it's going to be possible.
Such as adding the following checkbox:
<input type="checkbox" value="ship0-" name="search" id="shipping">
Then performing a search for Xbox that's 10% off and free shipping results in the following string:
http://domain.com/s/term=Xbox&<font color='"#FF0000"'>search=10off</font>&<font color='"#0000CD"'>search=ship0</font>
The Red and Blue lines need to be merged in order for it to work, like: