I just received an offer on my credit card (Barclays Aviator, if needed to verify). The offer ([Flight Cents](https://www.aviatormastercard.com/servicing/flightcentsinfo)) is as follows:

1. I can choose a limit every month ($1 to $500)
2. Every transaction during a billing cycle will earn points (American Airlines miles) as usual
3. In addition the cents will be rounded up to the nearest whole dollar (For ex, on a `$3.20` purchase, `80 cents` will be rounded
4. At the end of each cycle, the total cents rounded up thus will earn 0.5 American Airlines miles per cent up to a max limit set in #1
5. I'll be charged the rounded cents as a purchase

So let us say I set the limit as $500 and I bought one cup of coffee in the whole month, costing $3.20, I'll earn 3 miles (since it is not an American Airlines purchase I'll earn 1X) and for the 80 cents that are rounded I'll earn 40 miles. So the total AA miles I'll earn = 43

My bill for the month will be $3.20 + $0.80 = $4. This is an amazing conversion (~10 miles per dollar) rate but I am skeptical. What's the catch? How is the airline/card benefiting from this?

*I am thinking if I use this card for every small transaction generating a lot of cents that will be rounded up that will be the best way to extract value from this. If I end up rounding even $20 I'll end up with 1000 miles and so roughly $10 to $12 in AA value. Likewise $100 will mean 5000 miles. This will be on top of X miles for $X.*