The One Stop Shop lets a seller register for EU VAT in a single member state and file one quarterly return covering consumer sales across the EU, instead of registering separately in every country it ships to.
There are two siblings here and they cover different sellers. OSS is for businesses established in the EU selling to consumers in other member states. If you are a US brand shipping directly to EU shoppers, the scheme built for you is IOSS, the Import One Stop Shop, which covers consignments valued at 150 euros or less. Larger consignments and certain product categories still need standard import handling, so the first job is knowing which lane your average order falls into.
With IOSS in place, VAT is collected at the point of sale and shown to the shopper as part of the total. The parcel then clears customs faster because the VAT is already accounted for, which removes the surprise fee at the door that kills repeat purchase rates. Without it, the carrier collects VAT plus a handling fee from your customer on delivery, and that fee often costs you the customer.
Shopify can collect VAT at checkout and produce the reports you need, but it does not register you for IOSS, file the return, or pick the right VAT rate per country. You still need an intermediary established in the EU to file IOSS returns on your behalf, because non-EU sellers cannot register directly. That intermediary relationship is the part brands usually discover late, so line it up before your first IOSS shipment, not after.
First, confirm your average consignment value and whether your catalog stays under the 150 euro line. Second, appoint an IOSS intermediary and get your registration number. Third, configure checkout to collect the correct VAT rate per destination country and display it in the total. Fourth, put the IOSS number on your commercial invoices and customs data so carriers clear parcels under the scheme. Fifth, reconcile quarterly: your intermediary files one return, you keep the records that back it up.