As my main job, I am a college math professor (W2 wages). Last summer (2018), independent of my main employer, I worked teaching math to high school students and was paid 10K for it. I received a 1099-MISC for this work with the money shown in Box 3. I've received 1099-MISC Box 7 payments (mostly honoraria) in the past and always reported them on Schedule C.
My impression based on researching Box 3 is that it is intended for oddball payments far from ones main line of work, and doing work similar to ones main job is usually Box 7. I am concerned that this income belonged in Box 7 and the IRS will fine me for not paying self-employment tax if I simply report it on Line 21 of the 1040.
It seems to me I have three options
File Schedule C, as if the payment was in Box 7. (I would not deduct any business expenses.)
Report the income on Line 21. In which case, what do I call it?
Contact the people who paid me and ask why they think this is Box 3 and if they would like to amend it.
Any risks, benefits or options I am missing?