Treatment of parcels posted in letter boxes

Any outgoing postal packet transmissible by parcel post, which from any words or marks thereon, or other external evidence, appears to have been intended for transmission as a parcel, and has not been tendered for transmission as a parcel but has been posted in a post office letter box, or otherwise than as provided for the posting of parcels, may, in the discretion of the Postmaster General either be detained and returned or given up to the sender, or if it is fully prepaid at the rate applicable to any other post, and admissible by that post, it may be forwarded as a letter, printed paper or newspaper (as the case may be).