Jessica,
If you haven't done so, I suggest you write everything out exactly as it happened so you have it in hand, ready to fax or e-mail to the insurance company, the commissioner of insurance for your state or whomever may need it in the event that you don't get the best answer back. If you write it while you are not in a complete state of panic, you will be less likely to forget details that might be helpful, plus you will be able to formulate your best rational and empathy inducing case. Point out that you made the payment in "good faith" and that the flub was due to the credit union issue, not your lack of submission of payment or even lack of sufficient funds.
This link might be helpful:
http://www.oci.ga.gov/consumerservice/Complaint.aspx
I understand your fear that the insurance company may try to drop you, but I think the circumstances of your situation, your previous payment track record and the fact that you acted in good faith should help. Just don't give up. I think insurance companies often deny claims simply because most people don't stick with it and follow up. Many lay people feel that if the insurance company says no---thats it---and it may not be so.
Stick with it and hang in there. Keeping you always in my prayers.