Paypal upload shopping cart button not working
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I have a custom checkout process and would like to submit individual items to Paypal rather than a single aggregated cart. Therefore I'm trying to use the upload button. So here's an example form that I'm submitting:

    <form method="post" action="https://www.sandbox.paypal.com/cgi-bin/webscr">
       <input type="hidden" name="business" value="business@domain.com">    
       <input type="hidden" name="cmd" value="_cart">
       <input type="hidden" name="upload" value="1">
       <input type="hidden" name="notify_url" value="https://site.domain.com/ipn">   
       <input type="hidden" name="currency_code" value="CAD">
       <input type="hidden" name="item_name_1" value="test">
       <input type="hidden" name="amount_1" value="1">
       <input type="submit" value="submit">
    </form>

Everything seems fine, I can even go through the payment process and complete the transaction. So I go on the business account to view the transaction and see that it's there and it's completed (so my input values seem to be correct). However, when I try to click 'Details' for the transaction I get a 'Sorry - your last action could not be completed' error page. And, more importantly, IPNs are not getting sent.

However, if I try different Paypal buttons then everything seems to work fine. IPNs get sent/received (so my IPN settings seem correct) and I can view the details of the transactions. These results are consistent. Been testing them in the Paypal sandbox area with a verified business account and a verified personal account. So it appears to be a problem specific to the upload button.

To change the Paypal button I was using I would simply modify the form above. For example to use a Add to Cart button I simply replaced the 'upload' field with a 'add' field and replaced the 'item_name_x' and 'amount_x' fields with single 'item_name' and 'amount' fields.

Sent a ticket to Paypal, but was hoping someone here might know what the problem/solution is.