Off-the-shelf covers 80%.
Every commerce platform gets you most of the way there. What differentiates your store — the tier pricing, the quote workflows, the multi-warehouse fulfillment, the custom shipping rules — lives in the last 20%. That 20% is where every store hits the plugin ceiling and has to make a choice: work around the platform, or extend it properly.
I extend it properly. That means add-ons that survive core upgrades, follow the platform's own patterns, and stay maintainable when you inevitably rev the base version.