When a marketplace supports this, you can use EffectConnect to put your orders as shipped with letter mail as the delivery method.
You can set this in your channel's configuration or through track & trace mapping. You configure a general delivery service in your channel, and the track & trace mapping is used when you use more than one delivery service.
To read more about setting up your channel, read this article. Want to know more about track & trace mapping, and how to best use it? Read this article.