Quality system certification is not handled, and it can only be obtained by applying to a third-party certification body and passing the audit of the certification body. If your company has passed the audit of a third-party certification body and is currently processing the certification certificate, you can show the audit report of the certification body to the customer, or ask the certification body to provide proof that it is processing the quality system certificate.
If you haven't applied for certification, you can only explain the situation to the customer truthfully, show the customer the certification materials of the current company quality system, and let the customer evaluate your quality assurance ability. The certification materials of these quality systems include your factory's quality manual, procedure documents, management system, quality records and so on. . . . .