Something similar happened to me quite a while ago. The person insisted that they had sent my packgae but it didn't show up, and they claimed that maybe it was delivered to someone else. They said they had a tracking number for it. I explained that I lived in a small rural area and knew the mail lady personally and she knew all of us, and she whould know who got my package. I asked for the tracking number and without an email or explaination, my money was refunded.
I definatly would follow up on this, because if she did it to you, she will do it to others. $30.00 may not be a lot to you, but it's the principle of the thing. She needs to not get away with being dishonest and she is probably counting on your not doing anything about this. You could check with your bank and see if they can return your check to her and reverse the $30 from her account. I think the deadline is 6 months or something like that. It's worth a try.