Shopify Cart Page Enhancements

I need my Shopify store’s “Add to Cart” page refined so it works smoothly and nudges customers toward a larger purchase.

Here’s what I’m looking for:
• Enable custom product options (e.g., size or engraving) that feed straight into the cart without reloading the page.
• Add tasteful upsell suggestions right on the cart page using native Shopify features—no external apps.
• Allow shoppers to enter and apply discount codes before checkout.
• Fix a current glitch where the “Add to Cart” button occasionally fails to register the item.

I’m fine with Shopify’s default look and feel as long as it stays clean and mobile-friendly. Keep the work inside standard Shopify functionality—no custom scripts or paid apps. Once complete, I’ll test the flow end to end; if every feature above runs error-free on both desktop and mobile, the job is done.