The one-off type of non-w-2 income can go into miscellaneous income. If this isn't a business and these types of payments are going to be infrequent, Miscellaneous income is the easiest way to do this.
A US company is required to send you early in 2019 a 1099 showing how much they paid you.
The estimated payments are used when you know that you will owe too much. There are safe harbors that can apply, but you get concerned when the amount you owe in April is more than $1,000.
The issue for your case is the fee of $5,000. If you are in the 22% tax bracket that would mean that you owe an additional $1,100 in federal taxes. So if you were getting a small refund or owed a small amount, this extra income could trigger the underpayment penalty.
You have several options:
- If you are sure that you will make the safe harbors, or that you would have had a sizable refund without this transaction, then waiting until you file your full return should not be a problem.
- If you might run into an underpayment issue, then either send in an estimated payment (due Jan 15th 2019) or if you can add some additional withholding to your W-2 income via an updated W-4 that will work also. The updated W-4 might be tricky to do before the last paycheck of the year, plus you need to undo the change early next year.