United price should not change when shipping method is updated

Problem/Motivation

We experienced a very annoying problem with Commerce Shipping. Hopefully this is a configuration issue that I am not aware of. If we change the price of a product variation, the unit price of that item on an order placed before the price change would update automatically once we change the shipping method for that order.

We are using Commerce 8.x-2.40 with Commerce Shipping 8.x-2.11.

Steps to reproduce

• On 1/1, an order was placed. SKU: abc, unit price is $1. Qty is 3. Shipping is 0.The order total is $3.
• On 1/2, we increase price of SKU abc to $2.
• On 1/3, we ship the order. We adjust the shipping method from Ground shipping to 2 Day shipping. As soon as we change the shipping and save the shipment, however the unit price on the order changed to $2.

Proposed resolution

This is not right. The order was already placed on 1/1. We increased the price after the order was already placed. But the price change affected the placed order? The unit price will not change if we had not changed the shipping method. However even if we change the shipping method, the unit price should not be changed. How to make the unit price stay unchanged once the order is placed no matter we change the shipping method or not?

Please advise how to fix this problem.

Thank you!