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.
AppStore 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
Website Purchase
If you purchase it via LawStack website you can request a copy there once you sign-in.
1.) Go to https://www.lawstack.com/ and sign-in.
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 support@lawstack.com