From my experiments with their online store shipping seems to be a flat rate, no matter how many in stock items I seem to place in the cart on their site shipping stays the same ($15 for regular, $35 for express). Maybe someone else can shed some extra light on the matter but it seems that's how they do things, with flat rates (please correct me if I'm wrong regular customers).

As for mixed orders (in stock items ordered with pre orders, or pre orders due at different times ordered together) LTC informs me that they hold all items till the last one in the order is with them so that they can send everything at once.

As for where they source their items, no idea sorry