The Bree Postmaster can grant Honorary Postman's Badges to individuals he deems trustworthy.
'Hail, ${PLAYER}. Are you here to help with the festival invitations?'
'Any person who handles mail in Bree must have a Postman's Badge, or face the consequences of the law. I can authorize trustworthy individuals by bestowing them with an honourary badge, but only with the agreement that it will be used for helping Percy in his current situation. Can I trust you in this?'
Speak to the Bree Postmaster to request a Postman's Badge, in order to assist with finding the lost invitations.