Do I need an EU warehouse to sell on Shopify in Europe, or can I keep shipping from the US with IOSS?

You do not need an EU warehouse to take a euro checkout. You need one the moment stock sits inside the Union, or the moment a customer return has nowhere to land. IOSS and an EU 3PL are different systems decisions. Do not treat warehouse location as a shipping-method toggle. This is operating mechanics, not tax advice.

IOSS for shipments from outside the EU

IOSS is for goods at or under €150 shipped from outside the EU. Checkout collects VAT. The carrier declaration has to carry the IOSS number so the parcel is not taxed again at the border. Over €150, or any shipment that is not leaving a non-EU node, is a different VAT path (formal entry, not IOSS). Get the IOSS registration, an EORI, HS and origin on the product, and a storefront DDP disclaimer in place before you collect. DDP is still three jobs: checkout collects, customs declares a value that matches what the customer paid, the carrier knows who pays. Those three do not travel together. Do not invent a rate.

EU stock means Union OSS

Stock inside the EU is local VAT. Union OSS is the filing scheme for that stock, not a second IOSS. The tax trigger is where the goods sit when they ship, not the language on the product page. Shopify will not let one shop combine Union OSS and IOSS when you have both EU and non-EU fulfillment locations. If you open an EU node and keep shipping some orders from the US, you have to split that tax setup. One shop cannot wear both hats. Shopify will push the shop toward OSS and can mishandle the US-shipped IOSS parcels. Operators who need both paths usually split shops or take the tax engine off Shopify.

US node or an EU 3PL

Keep the US node and ship DDP while one carrier stack, one IOSS number, and one US returns address still work. Stand up an EU 3PL when you need EU carriers, in-Union delivery times, a marketplace that will not replenish from the US, or a returns address customers will actually use. The moment the node exists you need an EU warehouse record, a filing country (an EU HS is tied to a country, not "the EU"), and a receiving location that can put a return back on the books. A Shopify refund with no receipt in the Union is invisible inventory. After Brexit, UK and EU stock are two divisions even at the same 3PL: different packing slips, different returns addresses. Set the owning entity's base currency before the first EU receipt. Rate updates do not revalue stock already received.