A startup owner selling dog toys seeks an affordable upselling app to recommend complementary products (like treats) on product pages or via pop-ups after adding items to cart. They specifically want suggestions displayed below the “add to cart” button or in a separate window, similar to a provided screenshot example.
Recommended solutions include:
Alternative approach: One respondent suggests SMS marketing for personalized upselling to returning customers based on their preferences.
The discussion remains open with multiple app developers providing recommendations. Most solutions offer tiered pricing or free plans suitable for startups, with features ranging from basic product recommendations to advanced bundle builders and post-purchase funnels.
You can try out HypeCart which is great for upsells and cross sells both in the cart and after the checkout. I think you will like it! 
We also have free plan.
You can find it here: https://apps.shopify.com/hypecart
Here is a video showcasing some features.
