Spreedly's docs do not mention anything. Edit: They do indeed. I plead illiteracy.
Paypal's docs say the card type is "MasterCard", but actually trying to authorize with that will get you a "Card type is invalid." response from Paypal.
What is the magic string you need to successfully charge a MasterCard? Examples I have tried which did not work are:
In case others come here looking for the other card type strings, here they all are:
After some trial and error, I found out that the correct answer is "Master".
This cost me some money, so I thought I would do some constructive venting by leaving some help for the next developer who wondered.
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