First of all, hearty welcome to our friendly King Community, where your fellow King game(s) players can help you with some of the questions.
Would you mind me asking what do you mean by you want to retrieve "CORNS"? The Farm Heroes Saga game does not have any Corn in it! Please give more details.
Now, as per the "Piggy Bank" -- We are NOT putting any effort at all to win those Gold Bars. King just added a feature of adding some Gold Bars into the Piggy Bank according to the Stars we gained on the levels we passed. So, you do not HAVE to buy the Bars from the Piggy Bank, we can always buy the "Shovel" boosters with the Magic Beans; And whenever the "Treasure Mill" event is available on the Mobile game, we always can earn / raise our Gold Bars collection.
In one of the King games (Diamond Diaries Saga) a feature of "Shake the Piggy Bank & get 5 Gold bars" feature is available. Let's hope that King will be able to bring that feature in all his games
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To add to what @Pummy_Raj said too, basically we are earning a special offer price and they are often very good deals and would cost much more if we bought without. I know a lot are confused from what I have read and seen on the forums and in groups sadly