When sending orders, or using returns with Royal Mail, you may notice that your orders have a "return address". This is essentially the sender address of the order.
The sender address is typically taken from Shiptheory settings, and will then apply that sender address to all orders sent. This can be changed by firstly hitting "advanced" in your main Shiptheory dropdown, then clicking "Shipping Location":
On this page you'll firstly want to make sure it's actually filled in, and from there you can make any needed changes to ensure that your sender address/return address is correct:
Please note that the address you enter here should match the address on your posting location setup with Royal Mail. This is a restriction put in place by Royal Mail due to the way they bill for shipments. If you want to make sure what is this address, you can refer to your Account Manager.
Once done, hit save, and that will update your default Return address.
However, some channels allow you to specify the sender address from them, rather than using the one in Shiptheory. Some of these include Brightpearl, Magento 2, TradeGecko, Shopify and the API. For more information on this, please see here: Shipping From Multiple Addresses
It's important to note that if you specify the sender address from your channel, that will be used instead of the shipping location filled out in Shiptheory.
If you have any questions or issues from there, please Contact Support.