I've ordered my first lightsaber and i've come across the urge to add a mystery box also. I live in the UK so doing a separate order just for a MB is not very viable (the shipping adds about 50% extra cost).
I have emailed the support email but as I am aware it can take time for them to come back I thought i'd ask the question here to see if this is something they they usually are happy to do or if once the order is placed adding items is also a no go.
Appreciate the feedback
I've added items in the past to orders, but its been a while and back in the day it was easier to get responses from customer service due to the far lower volume. But if you can get someone to respond to you, they generally are happy to add items to your order.
Make sure when you contact them that you include your order number in the subject field and it wouldn't hurt to put in the subject that you wish to add a MB. Something like "Inquiry: Would like to add Mystery Box to order xxxxxxx" OR something like that.
And keep trying. If you can catch them before they ship it, you shouldn't have any issues.