If you purchase your postage online, you do get a discount over the USPS counter rate.
Plus its so much easier to do than to stand in line each day.
Since I don't accept DC, I use Paypal shipping for all of my postage simply because that's where the purchase payments are. The only time I use Etsy shipping is for USPS 1st class international mail as Paypal doesn't offer it. Etsy shipping is charged against your Etsy bill.
Since you already know the weight of your packaged items (you needed to know that to determine your shipping costs) all you need do now is simply enter that weight into either the Etsy shipping label or the Paypal shipping label, select the package type and print the label.
You may find this thread on shipping for US sellers from fellow Etsy seller Goto to be useful
http://www.etsy.com/teams/7722/business-topics/discuss/10577585/page/1