Is there a fine for not notifying immigration in time when applying for a 30-day extension in Thailand?

I think you're confusing a couple things.

As long as your current entry stamp is valid (tourist visa or 30 day visa exempt) you can get a 30 day extension for 1900baht.

BUT

IF you're dealing with an immigration office that is hard core about a Tm30 form being filed on you within 24 hours of your taking up residence where ever you're staying AND they want that receipt when you apply for an extension you could be fined if you or where ever you stay didn't do it.

The fine for that is usually 800baht.

At least at the Bangkok immigration office they are not asking for proof of a Tm30 when you extend a tourist visa or a 30 day visa exempt entry by 30 more days.

You have a stamp in your passport that you are allowed to stay for 30 days, you like to stay longer and therefor you need to visit the Immigration Office before the last date of that stamp and apply for another 30 days. If you go after the stamped date the fine is 500 Thb a day.

Pretty sure as long as you do it before your current visa expires your ok.

But someone will know for sure