You can most likely book it through reputable OTAs such as Expedia with no problem but make sure that the fare class is correct; i.e. it is a fare class for premium economy - P is for standard, S and T for flexi (starting January 20, 2018, currently they're all just PEY). Note, that in case of issues with the booking such as re-booking or cancellation or other changes you will need to contact the OTA. Also, the OTA may be booking this as 2 separate tickets (DEL-SIN-DEL with a SIN-SFO-SIN booking inside) so make sure you have a good buffer between flights.