If I have, say, 20 copies of my game produced by TGC, but I want to send you some custom components I've made myself, (they're Ghost Meeples that match the artwork of the game), would I be able to send you a box of those components (x20) that you could add to the boxes before you sent them to our customers?
For general sales, no.
TGC only offers this service if you're fulfilling a crowdfunding campaign through them. In that scenario, they allow you to mail them components, and they can add them to the game for a small fee.
Zovu's answer is correct although it isn't limited just to crowd funding orders. As long as you use our Bulk Order Fulfillment (BOF) service, this is something we can do. You can learn about it at https://www.thegamecrafter.com/help/bulk-order-fulfillment. Send an email to info AT thegamecrafter DOT com and we can assist you further with this. Thanks