I just found the following video which helped me, but it wasn't ALL that I needed...
https://www.apple.com/105/media/us/apple-card/2020/53f631e5-305f-4ec8-8eb6-057df67c4579/films/how-to/make-a-payment/apple-card-make-a-payment-tpl-cc-us-2019_1280x720h.mp4
when I logged back in and clicked on the 3 dots in the upper right, I clicked on "Monthly Installments", then, I clicked on my most recent purchase in the list, at the bottom in blue it says "Pay Early", I clicked on this and it allowed me to make another payment after having just made a minimum payment.
I hope this works for both of you as well as it worked for me!
Sincerely,
Cyndi