When doing Merchant Fulfilled batch, sellers usually prefer USPS, who for books charge a media mail fee of about $2.80. (If you use some other channel, you can set a different price in that case.)
To add Media Mail Fee, kindly follow the steps as follows:
1. Go to Settings > Listing Settings
2. Scroll to the bottom of the section to the Listing Settings section.
3. Here, Add the amount that you want to be included as Media Mail fee as base fees along with the additional fee per pound, select the checkbox under 'YES' if you want the amount to be included by default while listing a product as Merchant Fulfilled, and then click 'Save' for this particular option.
This shall save the media mail cost and add it as you list products in a Merchant Fulfilled batch.