I'm thinking of making estimated tax payment for the 2016 return through plastiq. However, as per form 1040-ES, the appropriate payment stub has to be sent along with the check, which of course is not possible through Plastiq.
Their CS rep said that just including all the required information along with the plastiq payment is good enough and also mentioned that others have successfully made tax payments (though not sure if he was referring to just the tax due at the time of filing or estimated tax payments).

Also, as per IRS website, the fees paid for credit card tax payments (through their approved portals) is tax deductible. Would the same apply for the service fee paid to plastiq?

Anyone else have any experience with this? Would really appreciate any input. Need to make the payment by tomorrow to be sure of getting the credit card charge posted in the next statement.