Firstly, click on your account icon in the top right. Then, when the drop down list appears, click Account Settings under your name.
This will take you Billing & Usage which shows all of your Subscription information, including how many podcast slots you have, along with any remaining monthly allowances you have.
Click the orange Manage Subscription button. This will open a small pop up window that will show you your subscription.
If you click Billing History, it will show you your past transactions.
Then all you have to do is click Download next to whichever transaction you want and this will download your receipt to your computer.