All purchases made via ReadCube should automatically send receipts to the e-mail address you used to create your account.
If for some reason you cannot find your receipt in your inbox, check your spam and trash folders. If you still cannot find it, the best way to get your receipt resent to your account is to access your Purchase History. Your Purchase History is available directly on www.readcube.com/account on the lower half of the page. From here, you can view the invoice in PDF form.
The invoice will appear in your web-browser and you can right click to "Save as.." Choose a filename and don't forget to add .PDF at the end of the file.
There's also an option in your ReadCube Desktop client. Just click on the gear in the bottom left (the preferences icon) and choose the "Purchases" tab. From here, press the below indicated "Email Receipt" button to have your receipt sent directly to your e-mail address.