If I file my income tax on April 15th and plan to pay with a check, it will take a few days for the check to be delivered. Will I face interest and penalties for those few days for the check in the mailing system?
When sending a tax return to the IRS by mail you need to have the envelope post-marked by 15 April. The fact it may take a week or more for them to receive it, open it, and process the return and check is not important and doesn't cause you to owe interest or penalties.
When US mail was the only way to file, some post offices stayed open until midnight for those last minute filers. If you are going to wait that long to take it to the post-office before they close on the 15th. mDon't drop it in a box on the street, or in front of your house.
Those who are filing electronically need to push the button before midnight on the 15th.
You must fully pay or overpay by April 15.
A postmark is good enough if it is sent by US Mail (which it has to be anyway since it goes to a PO box). If you're mailing past about April 8, make sure it gets a postmark (not everything does). I recommend handing it to a counter.
If you are not ready to file, you can just pay using Form 4868, which is the correct payment slip and also grants a filing extension*. I like to overpay by a margin just in case I missed something; IRS is very reliable about giving refunds.
* The 4868 filing extension is fairly meaningless. If you still owe after April 15, you're still in trouble. If IRS owes you, the penalty for late filing is $0. The only other penalties are
- you don't get your refund until you file, obviously
- After too long, electronic tax services like TurboTax stop supporting that tax year and you must then hand file.