And now another discovery: even if I accept the checkbox that appears by default (Would you like to use your WHATEVER_I_PREVIOUSLY_USED card ), the charge still does not go through and gives the same "Must provide source or customer" error.

So, it would appear that currently once a customer has made a purchase with a credit card they can never make a purchase again with either their previous card or a different card???

Maybe this is because I am working in test mode? If so, it's unfortunate that I can't really test things fully and be assured it will work.