Support hiring splits into two situations with different rules: something is broken now, or you want someone to stop things breaking.
If something is broken now, describe the symptom, not your theory. "Our theme is broken" gets a discovery call. "Since installing a reviews app on Tuesday, the product page add-to-cart button doesn't respond on mobile Safari but works on desktop Chrome" gets a diagnosis, often in the first reply. Include when it started, what changed immediately before, and whether it's consistent or intermittent — intermittent points somewhere different from consistent.
Duplicate your theme before anyone touches anything. Shopify makes this trivial and it costs nothing. It gives you a working rollback and lets a developer test fixes without risking the live store. Any developer who starts editing your published theme directly on an urgent fix is telling you something about how they work.
Check whether it's your problem at all. Before paying anyone, check Shopify's status page and the status pages of the apps involved. A meaningful share of "urgent" issues are platform or app incidents that resolve on their own, and you'd rather know that before commissioning a diagnosis.
For urgent work, ask about availability explicitly, not responsiveness. "We respond quickly" is not a commitment. Ask what hours they cover, what their realistic start time is, and whether they charge an emergency premium — most do, typically 50-100% above standard rates, and that's reasonable for genuinely urgent work.
For maintenance, define the recurring deliverable. "Keep our store maintained" is unquotable and drifts toward whatever is loudest. "Monthly theme and app updates in a staging theme, quarterly speed audit, uptime and error monitoring, and up to four hours of small fixes" is a retainer you can evaluate.
Ask what they leave behind. After an urgent fix, you want to know what caused it and what prevents recurrence. A developer who fixes the symptom without explaining the cause guarantees a repeat visit.