Your customers can request a remake on an order that's been marked shipped order in Allmoxy by using the 'request remake' button in the bottom of the side bar.
When they do this it requires them to put in the quantities they want and reason for the remake but allows them to change nothing else (so they can't ask for a remake but then change the profile or finish options on you!) The 'remake' order will also have a total price of $0 so it won't create a new outstanding payment for them.
Is your B2B customer not finding the Request Remake button?
B2B customers can't request a remake from within their own instance, but they can long into their customer account on your instance and request a remake there.
Advise the customer to go directly to your instance, login to their customer account, then selecting the Request Remake button on their shipped order order.