Guess you are doing a “stock re-order level”.I do not know how you have set up your DB, i.q. is it normalized (in 1 collection you define stock and re-order level, in another you put your sales, and you calculate current stock by summing all sales, subtract it from stock) or denormalized (with every sale you adjust current stock in first collection). Mind you that for the last to work flawlessly, you need an ACID-compliant DB, which is not what Wix is offering.
If you do not care, you just write every sake to a collection, check/calculate stock on the onAfterSave and on screen you .show() a hidden box/textbox that says “Problem: current stock is below [re-orderlevel], please order more goodies.” or whatever.