Could it be that the dim weight of the combined order + free gift bumps up the shipping cost? Because I know that I have ordered items from China that have free

Could it be that the dim weight of the combined order + free gift bumps up the shipping cost? Because I know that I have ordered items from China that have free shipping unless I have more than 4 or 5 items in the same order, then the shipping goes from free to an unattactive amount. Sellers have told me to instead place multiple orders for smaller quantity and then the shipping on each is free.