GST charge on Mobile phone @12% instead of 18%

Dear all, 

please help me in solving this. 

GST rate on mobile phone was changed from 12% to 18%.

But the client who is selling the online mobile phones on amazon and Flipkart has charged 12% get instead of 18% (which is a new rate to be charged from 01-.4-2020) for last 2 months may and June. 

now how should i report this in GST. 

should i go with the current rate or the rate the client has charged. 

Regards

Sanjeev

pcs and trademark agent

7011624756