Where can you purchase international return postage?

I am currently in Korea and I am trying to submit documents to the Korean consulate in Atlanta so I can get my E2 visa processed without having to go to Japan for a visa run. The Korean consulate requires, in order to return my passport and documents to me, a self-addressed envelop with prepaid postage. I went to DHL and FedEx and both couriers were unable to offer prepaid return postage. Can anyone recommend a courier that offers this service?

PrePaid Postage

I had experience w/ the Consulate in Chicago. They would mail my passport back to me if I had an account with UPS or FedEx. They could just use the account no. and print out the shipping label. See if Atlanta has the same.