All of your past purchases for Coins and Real Money are shown in your “My Transactions” tab which you can get to via the menu dropdown when hovering over your username in the top right. You can filter your transactions in various ways to make it easier to find transactions you want to view.
If anything is unclear or you have transactions on your account that you think were not made by you. Please contact support immediately with the reason and transaction ID so that we can deal with this issue.
If you are having trouble finding your transaction history page you can click here to access it.