Error: Order Products must have a Price Book Entry that belongs to the Price Book related to the parent Order.: Price Book Entry ID

Using default checkout for testing purposes

I am able to get to the cart to order action, which throws this error.

The product I have in cart is in a pricebook (**the store pricebook**).
This pricebook is connected to the user's buyer group as well.
The product also is in the **standard price book.**
For testing purposes, I also added the "Shipping charge for this delivery method" ghost Product to the price book to no avail.

3 Questions:
- Any input on why this is happening? Previously seen before?
- How does a cart associate the order to the correct pricebook during the cart to order action?
- Is the "Cart to Order" action source code available somewhere? I believe it's built in and that the answer is no, but it'd be helpful to know how it works internally instead of dealing with a black box.

Thank you,
Pierre-Alexandre

Start by reproducing the error through the default checkout flow, from the cart to the Cart to Order action, and inspect how the store pricebook, buyer group, standard price book, and shipping-charge product are configured. Done means identifying the cause of the Price Book Entry mismatch and documenting whether the Cart to Order source or its pricebook-selection behavior is available.

Written by the indexing model from the issue text.