Your customers may need to speak with their banks if they haven't requested activation of their cards for international transactions, since the transactions may be flagged otherwise. This is a simple call; shouldn't take them more than 5-8 min depending on the bank.

In addition, 2-factor authentication (2FA; Verified by VISA; Mastercard Securecode) is mandatory for debit card based transactions. I don't think that's the case for credit cards, but I wouldn't be surprised if, in either case, banks decline to approve transactions from payment processors who don't support 2FA --> so perhaps this is something you could check with your payment processor?

paypal: Many banks in India don't allow transactions with Paypal (perhaps because of concerns of paypal's compliance with Indian banking guidelines), so Paypal is hit or miss.