After purchasing an asset, you will receive an email containing a link to the receipt. However, if the seller is connected to PayPal, you will receive a separate email from PayPal containing the receipt.
If the seller is connected to Stripe, you will be able to access your receipt by going to: https://www.behance.net/USERNAME/purchases.
If the seller is connected to PayPal, they can export a PDF of the transaction by going to their PayPal dashboard, finding the transaction and using: More actions > Print.