How changes are made without breaking updates
Hooks first, template overrides second
WooCommerce templates are built from actions. The single product page, for example, is assembled by functions attached to hooks such as woocommerce_single_product_summary at set priorities. Moving the price below the description is a matter of removing one callback and adding it back at another priority. No file is copied, so there is nothing to go stale. We override a template file only when the markup itself must change, and then we copy the single smallest file into the woocommerce folder of the theme, keep its version header, and note it in the project documentation. After each WooCommerce release, the system status screen tells us which overrides need review.
Where the code lives
Behavior belongs in a site-specific plugin, presentation in a child theme. Pricing rules, checkout validation and order data are business logic, and they should keep working if you redesign the site next year. Each customization gets its own class or file with a clear name, and can be switched off without touching the others. That structure is what lets us answer the question "what changed the total?" in minutes.
Checkout fields, done for the checkout you run
On the classic checkout, fields are filtered through woocommerce_checkout_fields, validated during checkout processing and saved to the order object. The block checkout ignores those filters and expects fields to be registered through its own API. Before adding a field we confirm which checkout your store runs and whether that is likely to change. Custom values are saved through order methods instead of direct post meta calls, which keeps them compatible with high-performance order storage.
One place for price changes
Price logic goes wrong when several pieces of code adjust it at different moments. We calculate custom prices at a single point, normally before cart totals are computed, and let WooCommerce handle tax, coupons and display from there. Rules are checked on the server every time, never trusted from the browser. If an existing extension already owns pricing, we extend it through its filters. We do not stack a second calculator on top.
What we test before release
Each change is tried against guest and logged-in customers, every enabled gateway, coupons, tax-inclusive and tax-exclusive display, and the mobile layout. We place orders and read the resulting emails and admin screens. Then the change ships with a short note on what it does and how to disable it.
