Send your full app list before asking for a quote. Every app on the store, what it's for and which ones you'd happily lose. A specialist who asks for this first is thinking about the whole stack, not just the app you called about.
Ask which of your apps they've configured before. Subscription, review, loyalty, search and email apps each have their own quirks with themes and with each other. Setting up an app they know is a fixed-price job; learning one on your time isn't, and a good specialist says plainly which is which.
Check they'll use app blocks where the app offers them. Well-built apps place themselves in the theme through app blocks and app embeds (the app's own sections and site-wide switches in the theme editor), which you can move or turn off and which disappear cleanly on uninstall. Some older apps, and some installers, paste code straight into theme files instead. Ask which approach they'll take and why.
Ask how they'll test the purchase path. A new upsell app can break the cart drawer, and a subscription app changes how the product form submits. A careful specialist places a test order with the new app live, checks that discounts still apply, and confirms each analytics and ad platform records the sale once.
Red flags. Installing a second app to fix the first. No plan for the code an old app left behind. A paid app recommended for something Shopify already does natively. And "it works on my screen" in place of a test order.
Get a stack document at the end. One page listing each app, what it does, where it appears on the storefront, what data it can access and who supports it. It costs little to write and saves the next developer hours of detective work.