I have a 2018 glc43 coupe. I have only drove 6000 miles should I do service b？（i only did the oil change last year while having 3000miles on it）

Varies by dealer. Call your local service dept for their prices. It’s a set price for each dealer. Shop around if there are multiple dealers near you.

Personally, I would keep to the annual service schedule even if you don’t hit the 10k miles to preserve warranty, increase resale value. Maybe every 14 or 15 months at most. Your call.

I second that. I always get the cars serviced once a year even though we only put 4000 miles on each of the cars annually. The dealers want you to anyway. I do the prepaid maintenance package so it's all paid upfront although some people don't like that. I find it is just easier. The wife can run a car over if I am busy and we don't have to worry about anything.
The services do vary from one dealer to the other. Just call them up and ask how much.

My first service at 10k miles was $300 even in California at a independent certified Mercedes garage who does only Mercedes .

I drive about 5k miles per year but always get the annual service. I use a local independent garage that specializes in Mercedes. I just got the Service B interval. It’s effectively the same as Service A but with a brake flush, and you might need a cabin filter. The difference is that my garage actually looks to see if you need the filter replaced versus just charging you for the full Service B. My experience at a Mercedes dealer for my rear diff oil change was less than ideal (and overpriced) so I researched and found a reputable garage. I have a 63S, and a Service A is under $200, plus a brake flush for under $150. Two dealers quoted my over $500 for a Service B, so about 30% less, but with a personal touch.