Meowsie,
I would have to agree that you should do exactly what feels right to you. NO ONE has the right to judge you. It isn't as if you pushed the fedex lady down and ran away with your meds...you tried to pay and there was a mistake. Contact them, don't contact them...I would say it is entirely your call and your call only. I can assure you that they aren't going down the tubes due to one mistaken missed payment (and not your mistake at that). Maybe you got the order that was free because it was all the 1-2 pills that get pinched out of so many orders...JK but you get what I am saying. I am sorry you have felt that your integrity was questioned...you didn't make the mistake, someone else did. These kinds of mistakes happen all the time, the difference is that WE are usually the ones out the money/time/medication. You attempted to pay and she COULDN'T take your money due to FedEx policy. I am guessing that just like all the rest of the emails that are sent, yours won't be addressed either.
Best of luck and yes, perhaps because you are the good person that you are, the universe saw fit to reward you.

P:)