Transmission service can mean a drain and refill, a pan and filter service, or a fluid exchange. The word flush is used inconsistently, so ask exactly which procedure is proposed.
The appropriate method depends on the manufacturer's instructions, transmission design, service history, and condition. No fluid procedure should be presented as a guaranteed cure for slipping or internal damage.
Contact Certified Transmission at 1303 Fort Crook Rd. N. in Bellevue to review your vehicle's needs. Discuss any symptoms before authorizing service, and avoid cleaning chemicals or additives unless specifically appropriate for the application.