Custom Shopping Cart Text Box Code No Longer Works

[kwerty]

I designed a site for a client some time ago and it's still in use. The client wanted a text box that would allow visitors to add a custom shopping cart item (and price) that wasn't listed on the site. When a visitor hit the submit button, it would then add the item to their cart. Code for the box is:

<form target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
    <input type="hidden" name="add" value="1"> 
    <input type="hidden" name="cmd" value="_cart"> 
    <input type="hidden" name="business" value="seller email"> 
    <input type="hidden" name="item_name" value="Add_Item"> 
                    <span class="style1">Item Description:</span>
    <input name="item_name" type="text" value="" size="42" maxlength="70">
                    &nbsp;&nbsp;<span class="style1">Amount: $</span>
    <input name="amount" type="text" value="" size="4" maxlength="7">
    <input type="submit" value="Submit"><p>
</form>

This has worked perfectly for years, but recently visitors submitting an item only ever see an error message from PayPal indicating that "things don't appear to be working at the moment".

How can I fix this?
Thanks.