at 15 min, you're changing oil, not completing a "service".
On the GC series, the initial 50 hour "service" includes, front axle oil change, engine oil & filter change, hydraulic fluid & filter change( Plus cleaning the pickup screen), fuel filter change. Most likely an air filter, a good grease job, checking coolant level & Ph, checking loader, backhoe and wheel hardware. Tire pressures, belts, safety switch operation, light & hazard operation.
That's just what I would do off the top of my head. I would have to go over a checklist for anything I missed.
$600 on the high side for a sub-compact. $289, thats a steal, I would bet thats just a fluid change, nothing extra. Pick-up & delivery or on-site travel plays in too.