Can I pay electronically after efile has been accepted even though I initially opted to mail a check?

Is this for taxes due for your Federal and or state?

If you already filed your return with the IRS, you can choose to make a payment option through the IRS. You can pay electronically here rather than mailing a check out.

For your state taxes, you can visit your state's website or by contacting your state Department of Revenue to see what options they have available for payment.