Thank you for playing!
The amount of coins is simply by how much you have beaten a level. Here's the math: (((TotalScore-LevelTarget)/OriginalLevelTarget)*10)+10
Example If the level target is 50, and you get 70: (70-50) = 20 / 50 = 0.4 * 10 = 4 + 10 = 14 Coins
You also asked in the rating "is there a way to delete cards?", when you reach 30 cards, newer cards are added to your collection, which then in every shop, you click on the Deck button to replace cards from your deck by new cards from the collection (Costs 5 coins).