The Quick Shipper Badge is automatically applied to your account when it has been shown that you ship your orders out, on average, within 24 hours of receiving payment from your buyer. You must have sold and shipped a minimum of 3 orders in the past 90 days for it to appear, as well. If you have an order that took a little longer to be shipped and results in the badge disappearing, don’t worry! Keep up the quick shipping and it’ll come right back!
You can always head to your Reverb Dashboard here and look under "Badge Status" to see the stats and active badges on your account.