My reading differs from yours. Simply because PayUSAtax, Pay1040, and ACI Payments, inc. paid the IRS for an endorsement doesn't make them "official." Those third-party payers aren't different from Plastiq, et al.
If you look at the
IRS Website one can see that the relationship goes beyond being entities that have simply paid for an endorsement. There's even language referring to these processors as service providers right at the top of the page:
You can pay by internet, phone, or mobile device whether you e-file, paper file or are responding to a bill or notice. It's safe and secure - the IRS uses standard service providers and business/commercial card networks, and your information is used solely to process your payment.
Additional information in the page such as the fees being tax deductible for businesses and payments posting as directly related to the US Treasury point to a more intricate/official relationship. Plastiq on the other hand is not part of this group and processes payments in a different way so I would say that there is a clear distinction there. On the state side, various states (NY, CA, etc.) also allow for tax payments to be made with a credit card and refer to the processors as service providers and name them as such on their websites.
I agree with others that have posted the suggestion to lower cash advance limits as an additional safety mechanism to be sure, but I believe that Chase is probably zeroing in on plastiq and tools like PayPal key, and people sending payments to P2 via PayPal for Freedom bonuses or to meet spend.