Well, if you purchase something from an online store that offers you bonus 2x or 3x miles or more for every $ spent than even if they refund your purchase on the same CC, you will still get the bonus miles. CO usually doubles the bonus miles if charged through CO debit or CC however CO miles take a long time to post. DL is much quicker. Infact I bought something from HD using DL AMEX CC and my bonus miles posted on my skymiles a/c even before the purchase was posted on the CC account!