Kindle billing logic doesn't prioritize credit for email charges

I have $60 of credit, which Kindle billing uses when I buy books. However, when I send myself files via email it tries to bill my credit card. In this case, it was an old credit card and I was denied the transfer.

Besides being annoying, it's also bad business for Amazon. The credit card charge would mostly wipeout the revenues from the email charges (a mere $0.60). If anything Amazon should be prioritizing the use of credit for the small charges, don't you think?