Using the word installment, it sounds like you have a payment plan. Those payments must be made until it is approved.
You can pay with your bank account or credit/debit card electronically since you cannot mail.
All options available can be found at this IRS Payments link.
If you already have a payment plan, you can call the IRS and ask them to roll the new amount into the old plan.
If none of the above works: the IRS payment phone number is 800-316-6541
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