Click on the Account icon and select Billing History in the drop down menu. Here is where you can see what your current pricing plan is, what it costs, when you'll be billed next and see all the past charges for your account.
To change the current plan click the Upgrade button to the right of your plan, or the big one in the top right corner, either one will work!
Under the current plan for your account is the billing history, with everything that's been been purchased in your account. You'll see the date of the purchase, an order number, a description of what was purchased and the price. If you need a receipt for any of your past charges, just click the View/Print link to the right of the charge and you'll see a page with a receipt and a button to print it if you need to. If you wish to save the receipt, first click Print, then change the printer destination to Save as PDF and follow the prompts.