Can a USPS Flat Rate Envelope Be Dropped in a Mailbox? – This is a place for the community to provide tips and advice on Can a USPS Flat Rate Envelope Be Dropped in a Mailbox?. This topic was created by Troy Scoggins and the tips are provided by the community. The tips you add here can be your own or referred from another site. The best tips are then ranked at the top when up-voted by members of the community.