You really don't want people to read the fine print, right?
First, lose the <strong>
Second, why not just make it white on white? Or light gray, if you insist.
<font size='1' color='lightgrey'>This price includes a discount if you complete the purchase online<br>(online discount does not apply if you complete via the call centre)</font>
Remember also that different people use different browsers. If you try to tune too finely how it looks for your own browser, you will have Unintended Consequences(tm).
Take a gander at this:
Hope this helps.