Go to Settings > Payments > Billing Information and click Change
Click "I'm a Business Owner"
Fill in your identification number
Keep "VAT Taxable" unchecked.
Choosing the document type
After entering your identification number and enabling receipt issuing, you can choose the document type for client orders:
Receipt -- a simple proof of payment. Client billing information is not required.
Invoice -- a full invoice. Client billing information is required. Available from the Expert plan.
You'll find this option in Settings > Payments > Payment Receipts.