post-paid 
adjective & adverb
(With reference to a letter or parcel) on which postage has already been paid: [as adjective]: use the post-paid envelope provided...
- They recently sent me a letter, with a post-paid envelope and a request to re-subscribe or at least write back and explain why I'd given up on them.
- Stacks of questionnaires are on hand for us to fill in and send to the Guildhall by March 14 in the post-paid envelopes that are also supplied.
- The DVD is beautifully packaged in a bag that doubles as a post-paid return envelope - the whole concept is well thought out.