Ask HN: Linking bitcoin payments to the rest of your software

I would like to set up a webshop that accepts bitcoin payments, but as far as I know there is no automatized way to check whether an amount has been received.<p>The thing I came up with is letting users enter the address they sent the payment from, but then someone else could also use that address and get their products for free if they time it carefully. How would you guys make this a smart-guy-proof system?