You can try In Cart Upsell app. With its help, you can offer the most relevant products in the cart. AI-powered autosuggest built in the system can automatically recommend the exact product a shopper is most likely going to buy next.
If you want something more than a product recommendation to boost your convention rates, you could create gift offers to make your products a tempting bargain to hunt.
What you’re describing is a form of upselling, specifically referred to as “product upselling” or “product recommendation.” It’s a strategy used to encourage customers to add more items to their cart before checking out.
In your case, suggesting a phone case when a mobile phone is added to the cart is a classic example.