my iPhone 8plus fixed

You are asking about two very different, separate items.

• Repair the iPhone: iPhone Repair - Official Apple Support
• Sell the iPhone and replace it with a different one

First of all, if the glass on the back on your 8 Plus is broken, it cannot be fixed. It will require a full iPhone replacement. If you have an AppleCare+ on your phone, the cost is less than if you don't. Apple will provide the same model and I suspect color you purchased when it was new. You'll have to ask if there is any option to get the phone in a different color since the entire phone has to be replaced.

See this link for prices in the US --> iPhone Repair - Official Apple Support

As you can see, with AppleCare+ it will cost $99. Without it will cost $399.

Schedule an appointment at your Apple Store Genius Bar.