Utilizing a Twitch Extension, this mod allows for viewers to buy in-game mail for streamers using bits.
To use the mod, you first need to enable the extension in your twitch dashboard (https://www.twitch.tv/dashboard/extensions), which is called "You've got mail - Stardew Valley".
Once the extension is installed, you can configure the extension, adding your own mail items.
The config page will also give you your TwitchID, which you will need to configure the mod to receive mails.
edit the config.json file and put your TwitchID in it.
Play the game and enjoy reading the mail from your viewers!