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 styloid process (or  styloid part of the squamous temporal bone) is a slender pointed part of temporal bone. It projects inferiorly and anteriorly from the inferior surface of the temporal bone.