Nordica Order Edit

Cancel, add items and the EU withdrawal button, by Variegata Nordica.

Customers cancel their order before it ships and get their money back at once, shipping included.
They add items to it and pay only for the new items. No extra shipping: it all ships together.
Optional: a different size or colour, or a new delivery address.
On their order page in their account, on your order page in your store, and right after checkout.
Unpaid added items never ship: the order waits on a hold, and they come off again after the hours you choose.
The EU withdrawal button: two steps, no login, a confirmation email. Free and unlimited.
One flat price. No fees per edit, per order or per address check.

Install it from the Shopify App Store, then open it from Apps in your Shopify admin.