When selling across multiple sales channels you need to be able to effectively manage all of your orders across each of your sales channels. You need to have a system in place for you to keep on top of all of them across every sales channel.
When you start your own Ecommerce business, you are going to need to sort out your shipping methods rather quickly. Planning out your rates, packaging and more to make sure that you are not overcharging or spending too much yourself to ship items.