Shipping discount code for item X when purchasing item Y

[trucranems]

Hey,

So I'm looking to offer a feature were if you're buying a certain item(X) and also buying item Y, then item Y ships for free.

Item Y is really small and can easily fit inside the box of item X, but I can't seem to get my shipping to understand that. So it's creating larger than necessary shipping costs for customers. So I'm looking to just have it ship for free, but not ship for free all the time as some people buy the item by itself.