Is this money laundering if - there's mutual consensus? - money only processed for payments and never for cash withdrawal? - we don't handle cash?
The best way forward for you would be to consult a lawyer from the place you're going to register your corporate entity in, and then the same thing with every place where you intend to do business with.
That's so you don't end up like mr. David Carruthers.
You're on the right track by looking at the ways in which the system can be abused. Things to watch out for: regular users (you're aiming for travellers, so a person that uses this year long should stand out), volume purchases (extraordinary amounts / numbers of cards) and so on.
Establish what you think are reasonable parameters and ruthlessly prune whatever lies outside that. (aka the paypal method).
Take a look at both your local legislation and Visa/MC (whatever you'll be using) guidelines. It might be that you'll have handle know-your-customer provisions like requesting physical ID's to verify their names. And you will be liable for any fraud/laundering where people fill "your card" from a stolen card, spend the money, and 2-3 months later the chargebacks start coming.
Cash doesn't need to be involved - if you can make payments "on behalf" of someone else, in someone else's name, then that's directly relevant to money laundering.
In any case my credit card is valid in all countries where I see a VISA or Mastercard sign, and I have never heard of ever receiving discounts for using it. If anything some places charge a premium just for handling it, even in my own country.
This just seems like you trying to look for a problem to fit your idea. Sorry about being blunt.