How to offer a free product for in-store pickup orders over a certain amount?

[no_robotocha]

We have a product in our store that we'd like to also gift to all customers who spend over a certain amount AND choose to pick-up the order from our warehouse.

We'd like this item to be added to their order so we can manage stock levels and create reports.

As shipping is added at checkout, I'm not sure how we can implement this. We have apps Flow and Mechanic, if they might be utilised.

Thanks!