A café owner asked me for an online store. She didn't want Shopify, a domain, a theme, or a $90/month bill — she wanted customers to message her on WhatsApp, ask what's available, and place an order. That's not a website. That's three functions and a spreadsheet.
Here's the whole thing: an AI that browses your catalog and logs orders over chat, backed by two Google Sheets tabs, for under $20/month. It's about 200 lines of Apps Script, and the three tools that do the work are below — verbatim.
Why function calling is the right tool here
A keyword bot ("reply 1 for menu") falls apart the moment a customer types "what pastries do you have under 50?". A language model handles that phrasing easily — but on its own it can only talk. To actually read your catalog and write an order, it needs tools: functions it's allowed to call.
OpenAI's function calling is exactly that. You hand GPT a list of tool schemas; when it decides it needs data, it returns a tool_call naming the tool and its arguments; your Apps Script runs the matching function and hands the result back. Three tools cover a storefront: list products, look up one product, log an order.
