Do I design my device correctly?
Design seems to be missing, but a few critiques:


overall BOM (bill of materials) cost seems high given how cheaply I can buy scales
no accuracy spec. In fact, no spec at all. How accurately will this weigh? Calibration? Drift?
no mechanical design (particularly important for this application, given the mounting of the load cell)
battery seems completely wrong. My kitchen scales take normal AA batteries with a life of several years, rather than a phone battery that will go flat in months.
no controls: how do I turn it on? Tare? Units? (some people may want Imperial units for cooking)
power saving when not in use: mechanical off switch or something else? Note that the standby usage of a default Arduino is very high