If you’re looking to do real-time stock level syncing as well as bundling/kitting, check out Trunk. We integrate with Shopify, Amazon, eBay, Faire, Square, Squarespace, and Etsy. We have a superb integration with Shopify as you can see from our app listing reviews: https://apps.shopify.com/trunk

https://trunkinventory.com

P.S. I’m the founder, happy to answer any questions