I am very familiar with Fedex Smartpost (as well as most domestic shipping methods). Smartpost is handed off to USPS for the final delivery. I also know that because of this, Smartpost can be delivered to PO boxes.
Since BBTS states that they cannot ship to PO boxes, and that the carriers can differ, that means that there is no guarantee that the final delivery is actually made by the post office.
I'm still leaning towards taking a leap of faith and ordering from Underground Figures. Priority Mail, estimated ship date is tomorrow, and their eBay store is very reputable. I'll mull it over at work tomorrow and make my decision then. Thanks for all the help, and apologies if I derailed the thread a bit