AbleCommerce Shopping Cart Software

Would there be an easy way to add a small order fee to orders under a certain dollar amount?

You can make use of pricing rules I guess.

Have a look at this thread viewtopic.php?f=42&t=13934
You can try to do something like this. One suitable location could be the CheckingOut event handler in OnePageCheckout.ascx.cs. This is the last location available before a checkout, if this gets called then it means customer can no longer change basket items. So in there you can see basket total, if required add custom charges and let the checkout proceed. Hopefully that will do the trick.