Convert email CSS to inline styles
When sending an email receipt, run the email through a process that will convert all <style> tags and imported stylesheets into inline CSS. This is what Campaign Monitor does to email templates before sending them.
This should happen after the ^^ tags are replaced in the email template.
This is hugely important to anyone wanting to use FoxyCart's generated emails not only because the safest way to use CSS in an HTML email is to use inline styles since many clients ignore <style> tags completely and also because the HTML generated by the ^^ tags contains some inline styles that cannot be overridden without using the CSS direction !important (which should never be used)
There are a couple of scripts I know of that do this already, one would just need to be implemented:
Tanner Netterville commented
I also threw together a python script for this here: http://github.com/rennat/pynliner
Mailchimp also does this, for obvious reasons: