OK ... first look at the 1040X line 16 ... if you did not send the payment yet then that line should be zero. If that line is not zero please change it to a zero and recompute the rest of the 1040X. If your amended return still shows a balance due on line 20 then send that amount by 4/18 and it will eventually be reconciled with the amendment (if you efile the amendment) or mail the payment with the paper 1040X.
If line 16 is a zero AND you now have a refund on line 22 then you don't need to send any payment and you will have to wait for the 1040X to fully process.
