You need to make a new class for the span tag and use it with single quotes:
2 Bottles for <strong>$47.90</strong> / $23.95ea <span class='mygreen'>##Save $8.00 </span>
Or you can try with single quotes:
2 Bottles for <strong>$47.90</strong> / $23.95ea <span style='color:white; background-color:green;'>##Save $8.00 </span>