BUT, my due date was August 10. I submitted be mail (which was received by Immigration at Chaengwattana on July 24) and have never received a response. If the worst is a monetary fine, I will just wait to see if it shows up in the mail this month.
I appreciated all of the suggestions. I followed Anisa Ismail's advice (thank you very much!) and found that my information had been duly recorded. Needless to say, I made a screen shot and printed it out in case I ever have a problem about this with Immigration.