Help with Amended

Help.
I filed taxes as a couple, I only filed my husband's W2 and forgot to add mine.
My husband's was accepted and the amounts were paid.
So I had to make an amendment to add my W2.
I had to return 1200 to the Federal Government and 754 to the State of Maine.
I have already paid the Federal Government, and the State is missing.
My concern is: I want to pay this voucher online, but I can't, because when I enter all the information for the amount, the system rejects it and says it is incorrect.
I would like to know if I can schedule a payment at the bank for April 15th and send the amendment form by mail.