Urgent WooCommerce Checkout Debug (Logic Fix Only)

My WordPress / WooCommerce site is already built and ready to launch. I only need checkout logic fixes, not new features or redesign.

Current issues

Products selected in the shop or upsell modal are not always added to the cart

Bundle pricing, cross-category discounts, and PDF add-on pricing are incorrect

Upsell selections sometimes replace existing items instead of adding to them

What needs to be done

Fix cart logic so all selected products (shop, upsell, bundle) are added correctly

Correct bundle pricing and savings calculations so totals match checkout

Ensure PDF add-ons attach to the correct product

Confirm checkout works end-to-end on desktop and mobile

Notes

This is debugging and logic cleanup only

No new features, no redesign

Site is on staging and ready to push live once fixed

Looking for someone who can start immediately and finish quickly