I am using the PC version of the game. If I go to Logistics or try to get a new Soldier the items available for purchase list the price in orders and in premium currency. How ever there is no way to choose to use premium currency. The only time using premium currency is possible is if you have no orders to spend.
Just as an example, if I want to purchase a frag grenade the price is listed as 1 bronze Order for Weapons or 20 gold (premium currency). Highlighting the grenade with the mouse cursor brings up a text field that says “you will receive one grenade” and no option to use gold is shown. If You have no bronze orders you can purchase with gold, but if you have any bronze orders, they will be spent without any option to use gold.