I was going to post this earlier & forgot, but today's email reminded me:
When you add a Mstercard payment card from your Masterpass Wallet to Plastiq you can make payments up to $500 with no Plastiq fees if the payment(s) is scheduled by May 31 and processed by September 30. If a payment is over $500 fees are charged just on the amount over $500.
Originally the offer was any payment(s) scheduled after May 15 & processed by Sept. 30, but I just got an email today that "Due to the high demand for our latest promotion" any payments scheduled after May 31 only get the first $250 fee free.
To get this promo, if you don't have a Masterpass account create one, and add the Mastercard you want to use for payment to your Masterpass wallet. Then log into Plastiq and click "Add payment card", a button appears for Masterpass, Log into Masterpass, select your payment card, and it returns to you Plastiq. Schedule your payments and it shows you no fee or fee amount if over $500, over $250 starting June 1.
I use Plastiq to pay my mortgage payment which is under $500, Plastiq fees are normally 2.5%. When I got the promo by email I scheduled my mortgage payments due 6/1 & 7/1 fee free. Now that they are limiting this I scheduled payments thru 10/1, all fee free. As my mortgagee is a small bank Plastiq prints & mails a check, so my 1st of the month payment is charged around the 22nd of the month prior to the payment. As the payment card statement cuts on the 20th and I get 3% rewards (combo of PayPal debit & credit cards), I maximize my cashflow and more than cover Plastiq fees. But now I have 5 months of fee free so I do better than cover my fees. 
If you're not familiar with Plastiq, it's a service that charges your credit card to make payments for you to companies or people that don't accept credit card payments - mortgage, rent, plummer, car mechanic, etc.
