Should I file an addendum if my return was already accepted by IRS and I just received an unexpected W2 for $180 dollars?

You cannot change or add anything on the return that you just e-filed, nor can you stop it.  It is too late, just like when you put an envelope in a US mailbox on the corner.  The IRS does not allow you to take it back.

If you left out a W-2, a 1099G, or a dependent, or a 1099 etc…DO NOT change your return while it is “pending.”  The changes will go nowhere.

Now you have to wait until the IRS either rejects or accepts your return.  If your return is rejected, you will be able to go into your account and make the necessary changes to your tax return and re-submit your return. 

 If the IRS accepts your return, however, then you have to wait longer until it has been fully processed and you have received your refund.  THEN you can prepare an amended tax return and e-file or mail  it in. You have to be able to work from that return exactly the way it was when it was e-filed originally.  You will need to use a form called a 1040X.

  Meanwhile, DO NOT go in and start changing anything on your return in the system, or you will make a mess for yourself.  Sit tight and wait until you see what the IRS does with the return you just e-filed

The Form 1040X you need will not be available until late February.

The $600 amount you saw was for them to give you a 1099NEC or 1099Misc for self employment.  Not for W2 income.  There is no minimum for W2.  And even if you didn't get a form you have to report all your income no matter how small.  Even $1 of bank interest you don't get a 1099INT for.

The analysis by @xmasbaby0 is correct on all points.

If you received the W-2, you should expect that the IRS will receive the W-2 information as well.

The W-2 can be added on a 1040X amended tax return but the amended tax return process will not be available for several weeks.

@tiffanie-tudor