That said, yours looks a lot more full-fledged than mine. Mine can only record transactions and then send a response (which it reads calculated from the spreadsheet based on the last transaction, which I'm very proud of for the level of customization it provides), but I've been noodling over how to add custom commands.
FTR, I shameless stole my idea from an earlier HN comment sometime last year; if you're considering implementing a SMS/spreadsheet system yourself, I'd check it out https://news.ycombinator.com/item?id=17313321
Edit: I really am happy with my solution because it basically lets you take any google sheets based transaction you have and add SMS as an input method.
Edit 2: Sorry if you encounter any rough edges if you go to use BYOB - I haven't marketed it widely, but my wife and I do use it. Any feedback is very much welcome.
Edit 3: If you don't want the manual import of transactions (but do want the freedom of a spreadsheet) then I'd suggest TillerHQ instead https://tillerhq.com I haven't used em, but I came across them when doing research before launching BYOB.
I also like that yours is generic -- I'm curious what other use cases might exist for an SMS based query system for sheets.
Another minor feature I added was lists so that my partner and I could keep our shopping list synchronized -- I could imagine it working well for that too
As for queries, I'd like to add queries to get the categories and query the amount left in a category. But that's more special casing - maybe it's a purist in me. I'm leaning towards letting people define a keywords area with three columns - the first is the word to trigger the command, the second is where anything after the keyword gets put (in some sanitized manner - like stripping whitespace) and the third would be the place to read the response.
I like the shopping list idea.
I haven't really taken the time to polish up some operational bits (we have really good logging/metrics where I work, and I miss it) or to market it though. So it's basically just my wife and I right now. I think the push to get something shipped (and my perfectionism) drained me, but reading this stories comments has been invigorating, and I an probably go about improving things in a more measured manner.