The bill payment service, Plastiq, has just announced a promotion in which you can pay your federal or state taxes with a MasterCard credit card with a lower than usual fee of 1.75% (Plasitiq usually charges 2.5% for all bill payments). To access this offer, use this link (this is not an affiliate link).
This offer is valid through April 18, 2016 at 5 p.m. PT.
For more details about Plastiq, please see: Complete guide to Plastiq credit card payments.
For more details about paying taxes with a credit card, please see: Top 5 reasons to pay federal taxes with a credit card or gift card.
Note that even though this rate is only slightly better than the 1.87% rate for paying federal taxes, Plastiq offers the additional benefit of allowing state tax payments as well.