I would NOT send the item until the check cleared either. Her making this fuss makes me wonder if she doesn't have every intention of withdrawing her funds because she'd know it was mailed and/or she might even have the item. Paypal/eBay isn't telling you to not ship the item for no reason, ya know?
I'd send her a link to this thread (okay, maybe not), but I would send her a link from Paypal's site directly to educate her on echecks and I'd look around to see if eBay has anything stated directly on their site. They used to, but I'm not sure if they do anymore or not.
I would also explain that according the eBay policy that you do have to ship within what is it, 5 business days? BUT, you have NOT received payment yet, so that time has not yet begun!!!
I am sorry that you may have to take a negative feedback. You however can do the same. With her low rating, it's going to look worse for her. If she's not selling though, I don't think it will matter much. If she negs you, I would respond professionally in your reply, but I would be sure to add seller didn't understand it takes time for echecks to clear and demanded I immediately send merchandise (or something to that effect). I would also clearly state the same thing on her feedback. If she responds to it, I'd counter it on her feedback (your own too actually if it goes that far). She gets the last word on her feedback, but I'd make sure to let other sellers know by responding but being very professional about it.
I'd also suggest adding her to your blocked bidder list so this can't happen again. At least not with her current username.