@Martingale Kim
Have you called the 1800 number first thing in the morning? I got one of those letters too for my 2019 partnership return, I mailed timely but due to covid the paper return wasn't processed timely and the computer assumed it was filed late. I called the number early in the morning and got through to someone within 10 mins. I explained what happened and I had a copy of my mailing receipt. I always pay for certified mail with tracking, best $5 I will ever spend. The IRS rep asked that I fax the mailing receipt. She stayed on the phone with me and confirmed receipt and immediately waived it. If you don't have proof of mailing, you should still call and explain the situation. This is happening to a lot of people, they are aware and may just waive it as a first time late filing courtesy, especially if you have always filed on time. They can see that when they pull your tax EIN. Definitely call before you pay. In the future, always do certified mail or e-file through an accountant. Good luck.