My recommendation is to write a letter and send the letter certified mail return receipt requested.
In the letter, explain that you in fact sent in extensions for each tax return; this assumes your extensions were timely.
Your facts don't provide when you sent the extensions, so that may be an issue.
Include a copy of what ever you have from the USPS to validate the date you sent in the extensions.
The letter should be factual, respectful and indicate based on the facts, you request the penalties be removed.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
