return, postage-paid envelope
A self-addressed envelope with postage already paid, typically enclosed with a letter or request so the recipient can reply without paying postage.
Please enclose a return, postage-paid envelope with the application form.
返信用封筒 is a more common term for a return envelope, but it does not necessarily imply that postage is prepaid. 返信料封筒 explicitly includes the postage fee.
Compound of 返信 (reply), 料 (fee), and 封筒 (envelope).