It is impossible to issue invoices directly on site. After the invoice is purchased, you will be given a small print sheet with a QR code in the upper right corner. You can issue invoices by scanning the QR code. Different from issuing invoices, you can fill in your own header. After trying it, you can only default your name and you can't change it. This is the method of invoicing in the on-site store. If you are in official website's case, see if there is this option on it. It is in the shopping order.

If you forget, as long as it is not too long, you can make it up.

I hope it will help you a little.