After upgrading Craft CMS, is there a receipt to print out? The in-app process of upgrading a Craft licence is smooth, but leaves no paper trail (not even a confirmation email). 
All I've found is a date of registration in the admin panel sidebar, without even stating the Craft licence type (client or pro).

Is there away to obtain a somewhat formal proof of purchase, like a receipt, that I can pass along to a client?
You should receive an email after purchasing a Craft license. Maybe double check your email details and/or your spam folder.

Anyway, if you email the guys at support@buildwithcraft.com they will sort out a receipt for you.