How to convert a American Express credit card purchase to EMI?

On the add-on card: yes, the primary cardholder is fully liable for all spends on the add-on. There is no separate credit limit or statement. So only issue it to someone you trust absolutely — spouse, parents. Not friends, not siblings you have money disputes with.

I've been a customer for 6 years. The biggest upside is the customer service. The biggest downside is the slow mobile app. If you do most banking on the app, test the app at a branch before applying — it's surprisingly old.

On the lounge access — the PassApp / Dreamfolks integration works smoothly only if you've registered your card on the lounge app before reaching the airport. First-time users always face issues. Do it the night before, not at the lounge counter.

Forex markup comparison: Amex = 3.5%, HDFC Infinia/Diners Black = 2.0%, Axis Atlas = 3.5% but with milestone fee waivers, ICICI Emeralde = 3.5%, Standard Chartered = 3.5%. If you travel more than twice a year, the math favours Infinia or Diners Black.