There are three services to pay your U.S. federal taxes: IRS Pay Your Taxes by Debit or Credit Card or Digital Wallet

pay1040.com 1.87% fee on credit (lowered from 1.99% on 01/02/2023). $2.50 flat fee on debit.
payUSAtax.com - 1.82% fee on credit (rate updated 01/03/2024 from 1.85%). $2.20 flat fee on debit.ACI Payments, Inc - 1.98% fee on credit. $2.20 flat fee on debit.

Many states also permit online tax payment; check with your state or this list from MasterCard.

The IRS has a system to view payments, and it's good practice to confirm all payments within a short time frame, so that any rare lost payment issue can be disputed.
Be mindful of time zones if paying on the due date as pay1040.com uses CDT timestamp and payusatax.com uses EDT timestamp.

In general, you're allowed 2 payments per processor above per type of tax (annual and quarterlies being 2 different types, for example). They're not billed as cash advance fees. If 6 payments is not enough to pay your bill you can use a service such as plastiq (2.25% fee). If making multiple payments, it is advised you join here to track your payments link , you will be required to give your banking information and will receive a pin via snail mail
(Confirmed 4/2018 in post #429)

Fees are tax-deductible for C-Corps but not individuals (2018 tax reform eliminated "miscellaneous itemized deductions"). The majority of people will not be able to deduct that expense, check with your accountant.

When making multiple payments at or near your credit limit multiple times, allow yourself 3-5 days between payments for the charge to show up on your card and your bank payment to clear. If you wait until April 15th to make payments, you will only be able to clear the first payment.

Best Credit Cards to use/buy cheap points:
- Any credit card to hit minimum spend and achieve signup bonus or spend thresholds.
- BOA Premium Rewards 2.62% Cashback (Card holder needs to be a Preferred Rewards Platinum Honors member)
- Chase INK Premier 2.5% Cashback on purchases over $5k (Points are not transferable to airline or hotel programs)
- Capital One Venture X 2X Cap One Miles/Points (now transfer to most airline partners at 1:1)
- Amex Blue Business Plus 2X Membership Rewards (capped at $50,000 spend per calendar year)
- Chase United Business Club Card, 1.5X United Miles
- BOA Virgin Atlantic World Elite 1.5X Virgin Atlantic Points
- Chase Freedom Unlimited, 1.5X Ultimate Rewards, paired with a premium card (Sapphire Preferred, Sapphire Reserve, INK Preferred, INK Plus)
- Chase INK Unlimited, 1.5X Ultimate Rewards, paired with a premium card (Sapphire Preferred, Sapphire Reserve, INK Preferred, INK Plus)
- Amex Everyday Preferred 1.5X Membership Rewards, (need to make 30 transactions in a month for 50% bonus)
- Amex Business Platinum 1.5X Membership Rewards on purchases over $5K

Big Spend Bonuses:
- Amex Delta Reserve, spend $60k get 30k bonus miles and 30k MQM
- Citi Hilton HHonors Reserve, spend $10k get free weekend night, $40k, Platinum Status
- Chase Southwest, spend $135k get Companion Pass (WN points are redeemed at $.011, @ 1.87% fee, you're essentially buying the companion pass for $847)
- Chase Ritz Carlton Reserve, spend $10k get Gold Status spend $75k get Platinum Status
- Chase World of Hyatt, spend $15k get one free night

Cash Back cards:
Elan Fidelity 2%
Citi Double Cash 2%

Earn Status/Elite qualifying points:
- American, Delta, Alaska, Hyatt

Pre-Funding allowed:
Amex Charge Cards

Pre-Funding not-allowed:
Chase

Quarterly tax due dates: the 15th of April, June, September, January