How to treat cost of merchandise that is printed on demand

My husband has an online retail business that is print on demand so he never has custody of the inventory, just pays the cost of the item when a purchase is made. I originally used to show it as purchases (line 36) as COGS but seems like this may be more like supplies? Just seems weird to put a big number on the supplies line when that is supposed to represent incidental small purchases. Would love some advice on the best place to show the cost of materials.