We also have a Plastiq guide on /r/DISchurners (which I've been meaning to update, but life keeps getting in the way).
I believe when FrequentMiler posted that article, Visas were still having a "cash advance" issue when used to pay a new Recipient for the first time on Plastiq. What would happen was when you set up a payment to a new payee ("Recipient") who you've never paid before and isn't in Plastiq's system, and tried to use a Visa card specifically, the payment would code as a "cash advance" which is not what you want because cash advances don't count towards minimum spend and incur fees. So, the recommendation was to "prep" the payment by (1) requesting the issuer lower the cash advance limit to $0 (or however much is the minimum allowed for your card product) so any accidental cash advance would be rejected, or (2) make the first payment to a new Recipient with an Amex, MasterCard, or Discover card (which didn't have this issue) to establish a "relationship" with the Recipient and get them into the system, then you could use the Visa going forward without accidentally coding as a cash advance.
Plastiq has since resolved the "cash advance" issue, so it's no longer necessary to "prep" a payment to a new Recipient when using a Visa.
If you're planning to use Plastiq, please show some support by signing up through a DISchurners link. You'll get $500 fee free dollars ("FFDs") after you've made $500 or more in payments. That means the next time you use Plastiq, they'll waive their fees for $500 worth of payments (saving you $12.50).
ETA: I'll add that even though Visa and Amex restrict certain categories of payments, e.g. mortgages, there are tricks that might work to get around the restriction.