It's more cost efficient to mail you a check for a penny, than it is to keep that credit balance on the books and mail you monthly statements for the credit balance for the next three, six, nine, twelve month, etc.

Also, if you take a look at the postage stamp on the envelope, you'll find that Capital One paid considerably less than $0.44 to mail the check. They negotiated a discount rate with the post office when they became the USPS's biggest customer by volume:

As of January, 2010, Capital One is the fourth-largest customer of the United States Post Office thanks to the countless mailers, offers and advertisements it sends to your home on a weekly basis.