Using flat rate commission calculations allow you to customize the commission amount on the individual product level. You will need to add the commission you would like to award your vendor in the "Cost Per Item" field on the product in Shopify.

This is the amount that will be awarded to your vendors when the product sells. This is a global setting so all products will need a value listed in the "Cost Per Item" field in order to award a commission. You can change your commission calculation type in your app, commission settings.