A bit of a good news and bad news for those who were waiting to hear the outcome of my call to Amazon's customer service... Was planning initially to make that call closer to release date but changed my mind and called earlier today.
I'm quoting verbatim: I apologize but we are unable to do any sort of amendments to orders that include unavailable items. That includes, adding the gift option, amending order, combining orders...
When I pressed for a why (since it can be easily done on other Amazon stores), the agent didn't have a justification.
However I was given the following advice (to be tested obviously): "once you see the steelbook back in stock, you can do either":
1- Try and edit your order to select "gift option" and the system might let you since the product is available.
OR
2- Place a new order and choose gift option etc... then cancel the previous order.
In reference to the second point, I reminded the agent about the limitation per order per customer and here's the good news for those who wanted to buy more than one copy for *cough* whatever reason *cough*

:
If 7 days have passed by after the first order, you can place a second one without any issues.
I trust this helps
