This is unreal, having the same problems trying to book with Air India Express. They don't take Amex and US Visa and MC are not going through. Guess I'll have to try Expedia. So silly.

EDIT: No go on Expedia.

Had to use Make My Trip for bookings with Air India Express & Indigo (they don't accept Amex. Somewhat hard to register for Make My Trip (but finally got it done). Once done, easy enough to book those airlines, including seats, etc. and one can manage the trip directly on the airline websites re add-ons.

Was able to use Amex directly with SpiceJet & Air India on their website (just have to verify your safekey passcode sent to your email). Much easier on these 2 airlines.