It is not immediately obvious, but you can pick up a "Mega Bundle" in the stores, which includes nearly all the DLC in one pack. This Mega Bundle is cheaper than buying all the DLC separately.
You can also buy DLC separately in smaller packs if you would only like to purchase one or two packs and not the full bundle. Check the stores for the full list of options!
Arrowhead do not handle anything regarding the store, sales or marketing. If you have further questions about pricing or the store you would need to contact Steam or PlayStation.