Code:


<form action="https://www.paypal.com/cgi-bin/webscr" method="post" onsubmit="this.target='paypal'; return ReadForm(this, true);">
        Size: <br>
        8 1/2 x 11<input type="radio" name="on0" value="Size: 8x11"><br>
        11 x 13&nbsp;&nbsp;&nbsp;
        <input type="radio" name="on0" value="Size: 9x13"><br>
        Custom&nbsp;&nbsp;&nbsp;
        <input type="radio" name="on0" value="Size: 11x21">&nbsp;
        <input type="text" name="on0" size="20"><br>
        <input type="hidden" name="os1" value="" />
        <input type="hidden" name="on1" value="" />
        <input type="hidden" name="os0" value="" />
        
        <input type="hidden" name="baseos1" value=" " />
        <input type="hidden" name="item_number" value="Image ID: <?php echo $row['image_id']; ?>">
        <input type="hidden" name="basedes" value="<? echo $row["cat_title"]; echo " - "; echo $row["image_title"]; ?>" />
        <input type="hidden" name="ba" value="USD">
        <input type="hidden" name="currency_code" value="USD">
        <input type="hidden" name="no_note" value="1">
        <input type="hidden" name="no_shipping" value="2">
        <?
            $radio = $_POST['on0'];

            if ($radio == 'Size: 8x11')
            {
                $amount = 35.00;
            }
            else if ($radio == 'Size: 9x13')
            {
                $amount = 45.00;
            }
            else
            {
                $amount = "TBD";
            }            
            
            echo $amount;
        ?>

        <input type="hidden" name="amount" value="<? echo "$amount"; ?>">
        <input type="hidden" name="item_name" value="<? echo $row["cat_title"]; echo " - "; echo $row["image_title"]; ?>">
        <input type="hidden" name="business" value="<? echo $row["business_mail"];">
        <input type="hidden" name="cmd" value="_cart">
        <input type="hidden" name="add" value="1">

        <p>
        <input type="image" ... align="left" width="55" height="15"><br />
    
        </form>  

Code:
    <?
            $radio = $_POST['on0'];

            if ($radio == 'Size: 8x11')
            {
                $amount = 35.00;
            }
            else if ($radio == 'Size: 9x13')
            {
                $amount = 45.00;
            }
            else
            {
                $amount = "TBD";
            }            
            
            echo $amount;
        ?>