Am I entitled to be paid for a trial shift?

Very likely, yes. Under the Employment Relations Act 2000 and the Minimum Wage Act 1983, if you perform work that benefits the business, such as serving customers, you are generally an employee for the time you work and must be paid at least the minimum wage. A brief unpaid observation may be acceptable, but a full shift performing regular duties is not a lawful unpaid trial. The employer cannot avoid the law by calling it a trial. Ask the employer in writing to pay you for the hours worked, and if it refuses, you can make a complaint to MBIE's Employment Standards team or bring a claim in the Employment Relations Authority. Keep evidence of the day, including messages, the hours you were there and anyone who saw you working.