gst inpact on replacement sales

This is the problem what E-commerce industries are now facing, Once the supplier has made the supply he would have to pay GST irrespective of the fact that the supply actually was not fulfilled, that's another thing that the supplier will get the GST adjusted in the next cycle and will actually have to pay the net amount only but the compliance cost will increse nonetheless. 

And yeah the repalced goods sent will be applicable to GST as well

A debit note will suffice the job