Processing the check is IRS's responsibility. If they accepted your check, but do not process it, it is their fault. Once they accept your check, your end of the deal has ended. A while back, I got a notice that I did not pay my state tax.
1 The xiphoid process (Latin: processes xiphoideus) is a small triangular cartilaginous extension of the breastbone.
The xiphoid process area is located at the lower part of the sternum. Pain in this area is known as xiphoid process pain, xiphoid syndrome, xiphoidynia, or xiphoidalgia. Xiphoidalgia is characterized primarily by tenderness and dull pain in the xiphoid process area. Xiphoid process pain is a subject of controversy.