Rate of GST in case of additional bed

If because of additional bed privided to customer, value of supply falls in higher slab, then at what rate gst should be charged?

For eg: In case where by actual tariff is Rs. 6000/- and the customer is also charged for an additional bed as required by him for Rs. 2000/- 

At Rs. 6000/-, the Gst should be charged at the rate of 12% 

But, the value of supply is Rs. 8000/-, and at Rs. 8000/-, GST should be charged at 18% 

What should be the actual rate of GST?