I purchase some MRP Product like weffer, biscuits from URD, in this case rcm is applicable and gst exempt or payable
purchase MRP rpdocut from URD then GST is applicable
Supplies received by a registered person from an unregistered dealer attract reverse charge liability: the registered recipient must pay GST on taxable supplies from unregistered suppliers and assume the obligations of the person liable to pay tax; MRP is not a GST concept but relates to Legal Metrology. Practitioners noted turnover linked registration considerations and a reported small day wise exemption for purchases from unregistered dealers. (AI Summary)
TaxTMI 

