Please be aware that extension is for filing the tax return - that is not an extension to pay.
So I suggest you to file your tax return before the extension expired. You will avoid late filing fees.
The next step - pay as much as you can and as soon as you can - that will help you to minimize late payment fees and interest on late payment.
You should consider financing the full payment of your tax liability through loans, such as a home equity loan from a financial institution or a credit card cash advance. The interest rate a bank or credit card charges and any applicable fees are usually lower than the combination of interest and penalties imposed by the Internal Revenue Code. If you cannot pay in full immediately, the IRS offers a short additional time to full pay, up to 120 days. Another option - an installment agreement would allow you to make a series of monthly payments over time. A one-time installment agreement fee of $105 will be charged when you enter into an installment agreement unless you choose to pay through a Direct Debit from your bank account, in which case the fee is $52.00. Taxpayers with income at or below 250% of the Department of Health and Human Services poverty guidelines can apply to pay a reduced user fee of $43. To request an installment agreement, when you are filing a tax return for which you are not able to provide full payment , you can request a pre-assessment installment agreement on current tax liabilities by using OPA. You may also submit Form 9465 , Installment Agreement Request, or a written request for a payment plan, attached to the front of your return.
Let me know if you need any help.
You will not avoid interest charges, however if the IRS agrees on delayed or installment payment - you will not be charged late payment penalties.