My name is Alex, I'm a guru here at Shopify!
If you have a collection created for a discount, there isn't currently a way to exclude particular items from having the discount apply. What I'd suggest is creating a new collection; you needn't display this on your site anywhere, it can be a hidden collection strictly to set up this discount. Create that collection so that all the products you'd like to offer the discount on are included, and then apply the discount to this new collection. There's more on creating collections in our guide right over here - using the 'does not contain' condition can really help in creating this collection, if you've just a few that you'd like to exclude.
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Alex | Shopify Guru
Okay, please tell me there's a way to exclude on sale items from discount codes. I run an ambassador program and they each have their own unique code. It will take a solid week to set it up the way you've explained below. There should be an easy way to simply unclick an item you don't want a discount code applied to. I can't get an external app for every tiny adjustment I need to make on my site. My apps are already running out of control. Has ANYONE found a simple way to manage this?