how to check if a customer has paid through paypal

Basically I have a third party shopping cart and I'm selling pdf files, th开发者_StackOverflowe shopping cart connects to the paypal shopping cart and the customer is then able to pay with paypal, I was just wondering how is it possible that I can check if a customer has paid with paypal and then redirect the customer to download the required file,

I'm building a query string with the quantity, price and the name of the item from my shopping cart and pass it through to paypal, the customer is then able to login to paypal and pay for it,

any help would be appreciated,

thanks

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That is described in the Paypal API and depends on what kind of API you are using.

After successfully paying, Paypal redirects the customer to your page, passing along some information, if the payment was successful, etc.

You can use this information to further redirect the customer to the PDF file or show an error message.

In the express checkout API, you have a parameter called "ACK". When this equals "success", then the payment was successful.