Start with migrations they have already shipped from your current platform. Moving from Magento or Salesforce is a different project from a first build, and an agency that has done it before will know where the data and redirects go wrong.
Check the partner tier in the Shopify Partner Directory rather than on the agency’s own site. Shopify awards Select, Plus, Premier, and Platinum tiers based on experience and results on the platform, and directory profiles also show reviews from real merchants.
Ask who stays on after launch. Some agencies hand over and move on; others run CRO, SEO, or marketing retainers. If you want one partner for build and growth, shortlist agencies that do both.
Shopify publishes Plus pricing in the UK from £1,950 a month on a one year term, or £1,800 a month on a three year term, with a variable fee for higher volume merchants.
Agency fees sit on top. Among the agencies on this list, published starting points range from about $25,000 to $40,000 for a Plus project. Migrations with complex integrations, multiple regions, or custom checkout work cost more and can take six months or longer.