When you use MasterPass on Plastiq it doesn't show your actual card number. It shows a MasterPass-generated card number. In my case it shows me a card number ending in 9xxx, whereas my actual card ends in 3xxx. But my actual card did get charged (on two separate 4250 charges minutes apart, which both waived the fee due the promo, since I actually wanted to charge $500 but the payee lets me do separate smaller charges and $250 is Plastiq's limit per transaction for the MasterPass promo).

So it uses the default card you have set up in MasterPass, but you have to go to MasterPass, not to Plastiq, to figure out which card is your MasterPass default card.

All Plastiq knows is that you're using MasterPass, not what card you have in MasterPass.