labour work

1. customer comes to repair a car to me with new denting part which he purchases from authorised dealer with gst bill rs 2500.

2. my labour charges is 400 rs to repair and get changes. I have to use gas and oxygen and rodes and nosadar to repair it.

3. my input part is 100 rs cost and 300 is my labour part cost.

4. what will my billing system.

a. I am registered with gst because
my firm is selling new tyres also more then 75 lacs in the name of ktr.

b. if I create other firm for labour work in other place in the name of rkt then and labour work is less then 20 lacs in a year. then what will be gst liability on labour work part.