This depends on your payment method.

For website payments (Pastela Cloud)

You can use the e-mail sent to you at the time of payment as a simplified invoice.
This e-mail is sent to the registered e-mail address of the pixiv account you used to sign up for Pastela Plus.

For in-app purchases

Apple sends a receipt e-mail titled "Your receipt from Apple" for each payment. This e-mail, along with the usage fee table below, can serve as a simple invoice.
Feel free to download the table for your reference.