Simple Shopify Functionality

I require a custom Shopify function that allows me to assign additional products to any product, specify a discount amount, which revises the main product pricing accordingly. For example: Shoe is $10, and the admin assigns Laces ($6) and Bag ($5) to Shoe, with a discount of $1. Then Laces and Bag appear as auto-selected choices on the product page for Shoe, and the Shoe price shows as $20 ($10 + 6 + 5 - 1). If any of the choices are unselected, the discount is removed, and the price for the main product (Shoe) is the sum of it and the choices that remain selected.

This can be built as a compartmentalized part of a theme, or as an app. You must have your own Shopify development environment. Please do not bid if you do not have extensive experience with building custom functionality in Shopify.