My 2015 S550 is now out of warranty. So I am no longer tied down to going to the dealer for Service A & B. My dealership charges about $1300 for a 2 - B service + 1 A service package.

I was wondering if I should continue taking my car into the deaership for service? Or would it be significantly cheaper to go to an Independent shop for B service? How much do independent shops normally charge for Service B?

Despite that I take mine still to the dealer, paying more what I think I should, I really don’t see why anything would go wrong if I took it to Costco for an oil change with OEM MBenz fluids in hand. It would be nice to do one oil change at least once every 1-2 years at the dealership to let them inspect it all around. However, the best solution is to find an independent shop that you trust. I’d estimate that fluids+labor it should not exceed $400 including oil and filters (if you buy them from FCPeuro or somewhere else and then pay the independent shop for labor).

You will save a bundle at an independent shop but be sure they work on Mercedes!
An experienced shop will know what things to look for.
They’d best have Xentry. They should stock filters and the correct fluids.
You need not be abused at the dealership for over 90% of what your car needs in service and repair.

Last time I took mine to the dealer, they performed a full inspection and an oil change for $230 (labor was $50)

IMHO, just change the oil & filter and coolant as recommended, pop the hood periodically and have a look around for possible problems and forget all the other MB inspection bs. That's all you need to do. This is what I've done with all my cars, including my '16 S550 coupe, and never regretted it. I'm 81 now so I've been at this for a while.

Do you have to use the dealership while the car is under warranty?

Absolutely not! You just need to keep your receipts to show that services have been done so that you don't void your warranty in case something breaks.

Find an independent shop that has long time experienced mechanics who work on Mercedes. Dealers mechanics come and go just like most of the other service and sales personnel. Most of them worked at other non luxury car brand shops before they worked at MB.

I found an independent that specializes in Mercedes. They were a referral from a client who has a 2000 S500, they are great. So friendly, car guys, have loaners. Cost wise, I was quoted $1,100 for the B service (40k and I paid that for the 20k) and he is going to do it for $570. Plus I am confident I am getting BETTER work. So no more dealer for me.

I'm going to the dealer currently, but that's because I have A VERY cheap service agreement for 10 years (5 remaining), called Mercedes Swiss Integral.

When this runs out, I'll probably keep going there and pay a premium, unless I find a reputable indie shop that can offer me the same place of mind.