Welcome to the community @MG82010
If they’ve not opted-in to marketing then, as you say, they shouldn’t be used in campaigns and certain flows. However, they are of value as they may become a customer/repeat purchaser!
The approach is to convince them to opt-in with incentives via data capture forms (e.g pop-ups), during checkout, and post-purchase. That requires split testing where possible to see what resonates with them and uplifts opt-ins. The incentives could be discounts, prize draws, access to helpful info (guides, product selectors/quizzes) etc.
I assume these contacts are purchasers and/or abandoned checkouts?
For the purchasers, I’d add a post-purchase flow with a very soft opt-in call to action. As they’ve not opted-in, it should primarily be to thank them for their order. I’d also focus on the copy around the checkout opt-in too.
And lots of split testing of data capture forms with various incentives!
One last point, do you have the Shopify bi-directional sync enabled to capture Shopify opt-ins to Klaviyo and vice versa?
No quick fix, but I hope that helps.
Regards
Andy