Order status pending until shipping can be calculated and added

How would I go about the following scenario:

Customer adds item to cart and proceeds to checkout
At this time, shipping charges have not been calculated or added
Customer “submits” order whereby it is placed “on hold” (no payment info has been taken yet) – Admin is notified
Admin calculates shipping costs and adds to order – Customer is notified
Customer logs in to account and completes checkout (PayPal) with added shipping charges included

It seems simple enough – can someone please steer me in the right direction on how to achieve this? Thank you in advance.

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