I would prioritize a partner that can explain the economics of your business and turn that into a specific plan. A familiar name and polished reports are not enough on their own.
Ask: Which problem would you address first? What evidence supports that choice? Which metric would change if it worked? For a Shopify store, the answers should account for margin, conversion, returns, repeat purchases and fulfillment capacity—not just traffic.
Then check communication in practice: who does the work, how often you review it, what requires approval and how problems are escalated. Ask for relevant references and inspect the scope, ownership of accounts, fees and handover terms.
A small diagnostic or tightly scoped pilot can reveal more than a large bundled retainer. I would avoid endorsing any named agency without verified evidence of its work for a comparable business. What outcome are you buying: new customers, better retention, or operational support?