How to add an automatic discount when customer purchases an add on product.

Hello, 

I hope this makes sense to everyone. I make surgical scrub hats and I am currently adding badge reels to my store that match some of my scrub hat designs. I have scrub hats on one page, and badge reels on another. I am wanting to offer an automatic discount when purchasing the matching set (or just a hat and reel in general). I have already added the badge reel to the scrub hat "add on" and the scrub hat to the badge reel "add on". That way when a customer purchased one or the other, they know I offer the matching set. Is there a way to add an automatic discount when a hat and badge reel are purchased? Or maybe a plug in I can use to make this possible? 

Thank you 

I agree. A feature like this would be very helpful!