not illegal. The tax law does not stipulate that the payment must be received before invoicing, but according to the transaction procedure, it is necessary to pay first, then pick up the goods and invoice. Legal basis: Article 16 of the Measures for the Administration of Invoices, units and individuals that need to use invoices temporarily can directly apply to the tax authorities in the place of business for issuing invoices on the basis of written certificates of buying and selling goods, providing or receiving services and engaging in other business activities and the identity certificate of the agent. If taxes should be paid in accordance with the provisions of tax laws and administrative regulations, the tax authorities should collect taxes first and then issue invoices. According to the needs of invoice management, the tax authorities may entrust other units to issue invoices in accordance with the provisions of the competent tax authorities in the State Council. Illegal invoicing is prohibited.