If I requested an installment plan, but want to just pay in full, can I still do this? It looks like the vouchers they sent me add up to more than what I owe.

Yes, you can pay in full at any point.

The total of your payments in an installment agreement will be more than the tax that was due because they charge a fee for an installment agreement and interest for the amount paid after the deadline. 

The sooner you pay the total tax, the less the total will be. 

How do I make the payment?

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@beth-cook1 wrote:

How do I make the payment?

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Go to this IRS website to pay federal taxes owed - https://www.irs.gov/payments

You can send a check or money order with the full amount due to the address on the payment vouchers or

you can make an online payment here

The Direct Pay option is free, the Debit Card or Credit Option charge a convenience fee.