Are there apps or ways shops can make customers have an open box for a designated amount of time?
For example, if I set an open box policy to be one week, whatever customers purchase within that week will be a flat rate/merged as one box instead of charging them separate shipping for each transaction.
I don’t believe this is possible. Each order will handle shipping separately. There’s no way to issue a customer free shipping on additional orders for X-amount of days after their first order.
The closest you could get to this is an app like this one (https://apps.shopify.com/order-merger) which can merge orders based on pre-determined factors, and then refund excess shipping costs (when applicable).