For purchases made after May 18, 2023, you can use LemonSqueezy's invoice generator
Purchased before May 18, 2023?You should be able to generate an invoice using your email receipt.
LemonSqueezy and Gumroad are both payment-processors used by software-developers to handle payments and manage licenses.
You may have heard of equivilant services such as Ko-fi, Patreon etc.
Learn more about: LemonSqueezy | Gumroad