I just moved my store here from another platform. My customers were able to have a shipping option that allowed them to place additional orders and combine it with a previous order that hadn't shipped yet, but have been paid for. This way they only paid for shipping once. This option was called Add to Existing Order on the other platform. Is there a way to do this here on shopify?
We have developed an app called Mergify. It helps you to merge orders. You can define all fields of the new merged order and check the app functionality by creating draft orders. When you are sure about your settings, you can make orders directly. Mergify notifies customers, cancels or archives orders, refunds extra shipping costs if you want.
You can define some rules to help it find mergeable orders for you. For example, you may not want to merge orders with a special tag, or only consider the last 48 hours orders. Mergify then shows you merging suggestions that you can take action with just a few clicks. Even this process can be automated, so if a customer creates orders with the same shipping address that passes your defined rules, Mergify merges it automatically.