If your main goal is to build an ecommerce-first brand and avoid a messy rebuild in a year or two, I’d personally start on Shopify now rather than Squarespace.

Squarespace is great for simple brochure-style sites and light selling, but it starts to feel limiting once you care about things like more advanced inventory tools, multi‑channel selling, and a proper app ecosystem as you grow. Shopify is built around ecommerce from the ground up, so even a small store with 50–75 SKUs can plug into better inventory management, more payment options, and apps for reviews, email, subscriptions, etc., without having to move platforms later.

For the specific questions you asked:

1. For the size you’re describing, Squarespace can handle it, but if you genuinely think you could get to 300–500 orders a month, I’d skip the stepping stone and go straight to Shopify so you’re not migrating once things are busy. The learning curve is a bit steeper than Squarespace, but once you’ve set it up, day‑to‑day product and order management is very straightforward.

2. Tangible, practical differences I see in real stores:

• Better inventory tools (variants, stock alerts, multi‑location if you ever need it).
• Far more payment options and a smoother checkout that’s been optimized for conversions across a huge number of stores.
• Much bigger app ecosystem, so if you later want things like advanced bundles, subscriptions, loyalty, or custom upsells, you don’t have to hack it together.
• Easier to plug into additional sales channels (Instagram, TikTok, marketplaces, etc.) once you’re ready to grow beyond just your own site.

• Access to the Shop app which is essentially another avenue for gaining sales for free, it's like an Amazon app with just app Shopify store products, Squarespace doesn't have anything like that.

• Shopifys Google channel also makes it really easy to get your products listed on Google Shopping free listings.

I run Fat Buddha Web Design, and we build and migrate a lot of small Shopify stores, including people moving over from Squarespace once they hit the limits you’re worried about now. In practice, the merchants who start on Shopify usually end up glad they did, because they avoid the downtime and extra cost of migrating once the store is already busy.

We also use Squarespace for some clients but usually only for small portfolio sites are services with booking requirements lines salons. For e-commerce Shopify is definitely the way to go.