I chose to pay the Federal tax through the mail when I filed. I found out they want SSN on check and I don't want to do that. How can I now pay on line instead?

Instead of paying by check, you can pay your Federal tax due at the following IRS website.

https://www.irs.gov/payments

Direct Pay out of your bank account is free.  They will directly debit your bank account.

If you use the credit/debit card option, there is a significant service fee.