cost of sales

For Services:
Of course, there would be no inventory. So there will be no need of dealing with stock measurements. Cost of sales would straightly be the cost of providing services.

For Nutrition Products:
in this case comes the concern of stocks. (How many products we bought + products which were in the closing stock of previous period – products which aren’t sold) x cost of products = cost of sales

you may incorporate the concept of lower of cost or nrv, but that would make this understanding far more complex.

for a service providing firm as you mentioned, there would be variety of stocks + services, each will be dealt separately, but amalgamated in one ‘cost of sales’ head