What productivity metrics to use in SMM and what in PPC for offline retail?

Question

contextual advertising

How to evaluate the productivity of the SMM department and how to evaluate the productivity of a separate traffic manager when working with offline retail?
Suppose there is a chain of stores under the brand "Delicious food and not so good", this store has a marketing and smm department, let smm be done beautifully and there are a lot of useful things for the client, a lot of likes and reposts, how to understand that these are not like-strokers on the other side of the Internet, but real buyers coming offline? How to evaluate and calculate the benefits of the SMM department? How to calculate the benefits of PPC? Suppose we launched an advertisement with Danone yogurt, which is at an unprecedented price in the stores "Delicious food and not so good", how to understand that the jumps of buyers are real from this advertisement, and not because an ordinary buyer saw a chip price tag and bought yogurts for a difficult winter period?
Share your approaches, cases, figures. How to solve the problem. What report should the department provide and with what figures.
PS There is no online store and online showcase in this problem.

Answer 1

2 promotional codes that must be registered on the site at the same time in order to take part in a draw or receive other bonuses.
The first promotional code is in the advertisement, the second is "under the lid" (there will even be enough promotional packaging). This way you can estimate the number of people who bought the product after watching the ad (of course, not everyone who buys after the ad will still register promotional codes, so the “registered” ones must also be multiplied by the coefficient k).