"Buy all Potties in the Pocket Dimension"
When you enter your Briefcase into the Different Hub, you will see that some Areas filled with Potties are locked, and you need 200 BC to unlock them.
You get BC by selling stuff, winning auctions or crafting items.
Buy all expansions and you will get the Achievement
Those areas also increase the Inventory Space of your Pocket Dimension, making it a VERY viable thing to buy early imho