I kept tapping the wrong card.

I collect miles the way a lot of people in Singapore do: a few credit cards, each with its own bonus categories, its own monthly cap and its own fine print. On paper I knew which card to use for what. At the counter, I didn't.

I'd forget which card earned 4 miles per dollar on what I was buying. I'd forget whether I had already used up that card's bonus cap this month. Sometimes I'd even forget which card in my wallet was which. The miles blogs explain the rules well, but nobody re-reads a blog post with a queue behind them.

So I built the tool I wanted. You add the cards you hold once. At the till you tap what you're buying, and MaxTheMiles tells you which card to use, how much cap is left, and the card's last 4 digits so you grab the right one.