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I don't know if I'd trust it. As of just about a month and a half ago, Plastiq itself started authorizing Chase cards as cash advances (they called it "a limit of 20% of your credit limit", but it turned out not to be so, but actually a cash advance authorization; Chase's default cash advance limit is 20% of your total credit limit, but you can change that cash advance limit to much lower).

They claim they'll do the actual final transaction as a purchase, but I wouldn't trust that if they're doing the authorization as a cash advance. And since this has only been in effect a short time, even if they're being consistent about this (which I don't trust either), any datapoints would have to be from just the last month and a half or so.

I had intentionally lowered the cash advance limit to just $200 on my Chase Marriott card, the lowest possible, so in my case the cash advance authorization for $800ish was just plain denied, so I never got to see what they did with the purchase. If your transaction posts as a cash advance, not only is there a cost/fee, but you also don't earn any points on it, and it doesn't count toward a signup bonus, which I would have wanted this transaction to do. So to me it was way too big a risk.

Now, I don't know how long this "temporary" policy at Plastiq was supposed to be in effect, and how you would tell whether it's been ended yet. (I only use Plastiq once a year myself, for some insurance payments.)