How to prevent duplicate PayPal payments?

Question

I have a simple Pay Now button, the code created through the button maker, with a "custom" hidden field added to identify it. I wonder if there is some additional hidden field I can add to tell PayPal that "This transaction should not be made twice".

<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">

    <input type="hidden" name="cmd" value="_s-xclick">
    <input type="hidden" name="custom" value="{MYUNIQUEID}">
    <input type="hidden" name="hosted_button_id" value="{MYHOSTEDBUTTONID}">
    <input type="image" src="https://www.sandbox.paypal.com/en_US/i/btn/btn_paynowCC_LG.gif" 
    border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
    <img alt="" border="0" src="https://www.sandbox.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">

</form>

As it is now it's unlikely that the customer should be able to pay twice, but I noticed that when the Sandbox had a glitch I succeeded in paying the same transaction twice. Something that I absolutely want to avoid. Not just for the customer, that could get refunded, sure, but also since I don't want my database messed up and I would like to avoid extensive coding to catch duplicate payments...

Answer 1

You would want to add a unique identifier to the "invoice" parameter; <input type="hidden" name="invoice" value="your unique invoice ID"> and enable "Block duplicate payments" within the 'Profile' > 'My selling tools' section on your account at www.paypal.com