MacBook Pro 13'' 2017 (A1708) after year of use showing 50% capacity I'am really shocked. Coconut battery shows 55.8% from Full Charge Capacity and battery is jumping in charge (like 90% after 5 minutes 50%). 

And some time ago was "Service Battery" label, but after it have disappeared. It's not ok, I understand. Have checked serial for replacement program and it's not for my laptop. My keys are ok, and I can't replace top case by this.

I was at authorized service (I'm not from country where apple devices officially, just authorized). There they can replace it only for ~500$. I don't know what to do.

Please help, maybe advice?
Shut down the laptop, then turn it on while holding the D key. It will open the Apple Diagnostics Test (https://support.apple.com/en-us/HT202731). After the test is done, the results may show if there's something wrong with your battery. If there are critical errors, I'm afraid that you may have to replace the battery. Is your laptop still on warranty? If it is, you can actually do it for free at an Apple Authorized Service Center, it doesn't have to be an Apple Service Center.