Since you are doing in-store campaigns, you want to measure the effectiveness of your Facebook ads through two means:

Actual performances on the ads themselves based on reach, impressions, and CPM. Keep in mind that you don't want to measure CPC, as you will be measuring performance through offline events.

Performance on ads through offline events: revenue per user and average order value per user.

For these types of campaigns, you want to make sure you measure your campaigns in two stages. The first will allow you to understand better how much you have to spend to reach a specific audience. The second will allow you to better understand the "direct response" performance on the ads based on actual sales data.