Should you use Shopify or build a custom store?
Shopify for the large majority of stores. Payments, tax rules, fraud checks and PCI compliance are handled and maintained for you, and none of those are things worth rebuilding or owning forever.
Custom becomes the right answer when the catalogue or the pricing logic genuinely doesn't fit a product-and-variant model - B2B tiered pricing, per-customer catalogues, configurable or made-to-order goods, or deep integration into an ERP. That's a real minority, and we'll say so plainly rather than sell the bigger build.


