How can I choose which location my client will pickup/drop-off their items from?

First up, set up multiple warehouse locations from your Company tab in your account settings. Once you've created all of your Warehouse locations, when you add your In-Store Pickup/Dropoff Logistics you will be prompted to select from a dropdown which Warehouse location the client will pickup or drop- off their items.

Here's an article on setting up your different warehouse locations.