It appears this is for Marketplace ad enhancements over the past year. Often times, credit cards will not be charged until the balance is over $6.00 and it appears to have taken about a year for that to happen with your account; the most recent charge being in March.

If you would like to review the charges, you are able to see them by going into the Marketplace > My Market > Accounting

There you will find a detailed list of your ads and any fees associated. Please let me know if there is anything else.