Selling product add-ons on Shopify

Infinite Options lets you bundle add-on products - https://apps.shopify.com/custom-options

An app like Infinite Options will be the best solution. There are also simple solutions that may work well for you if you don't have many products or frequent product updates - 1. Use product variations, each variation including a bundle of products. 2. Add checkboxes on your product page allowing bundles. You'll need some code work for this.