We've got an online shop with 5M daily requests. We figured out it was better to build the tech in-house to control the entire UX– custom shop, checkout flow, exchanges, labels, inventory speed and rotation.
Next step was moving all the fulfillment, purchasing and BOM parts over to a proper ERP system, like Odoo. Apart from one developer time, it costs us under $100 to run (the CDN costs the most).
Now we're pulling everything together under one UX. Sure, it's not for everyone, but it was fun to build and it's been a game-changer for us.