After you have added a Facebook Pixel code to a florist site, you can now add in the the script to track the Purchase event coming from Facebook.
In the header you will have already pasted the Facebook Pixel Code for their account.
Then, open Settings > General > Advanced Options
Switch to source mode and paste in the following:
<script>
fbq('track', 'Purchase', {
value: getTotal(),
currency: 'EUR'
});
</script>
The getTotal() function is what has been added to that page to return the value of the order. The above example is for Feehilys using EUR, so change to GBP for any UK florists.