In every purchase, there is actually a receipt that will be sent to your email. But sometimes it goes to spam emails and gets lost somewhere. But no need to worry, you can still request a copy depending on the type of your purchase.
App Store purchase is an Apple transaction. So, to request a copy, you need to contact Apple Customer Support. Details here: https://discussions.apple.com/thread/7539471
If you purchase it via LawStack website you can request a copy there once you sign-in.
1.) Once you signed in, please go to 'Account'
2.) Then go to 'Order History'
3.) View details of your order.
4.) And finally, request an email copy.
And you're done! If you have further question contact us at firstname.lastname@example.org