Can a supplier on electronic commerce opt not to register under GST

No. In terms of section 24(ix) of the CGST Act, 2017 irrespective of the threshold limit the supplier who supplies goods or services or both, other than supplies specified under section 9(5) of the CGST Act, 2017, through E-commerce operator who is required to collected tax at source under Section 52 is required to obtain a registration compulsorily. However, in case of supplier where the consideration with respect to supplies made on electronic commerce is not collected by the electronic commerce operator, then in such case, such supplier will be eligible to claim the threshold benefit for registration as per section 22 of the CGST Act, 2017.

(ICAI FAQ PUBLICATIONS 06-09-2017 TAX PAYMENTS,INTEREST, PENALTIES AND OTHER AMOUNTS)