i have created a form for payment and retrieved data from database to the input fields. my html is below:
<table>
<form name="postForm" action="form_process.php" method="POST" >
<tr><td><input type="hidden" name="txnid" value="<?php echo $txnid=time().rand(1000,99999); ?>" /></td></tr>
<tr><td>amount</td><td><input type="text" name="amount" value="<?php echo $_GET['id'];?>" /></td></tr>
<tr><td>Name</td><td><input type="text" name="firstname" value="<?php echo $user_name; ?>" /></td></tr>
<tr><td>email</td><td><input type="text" name="email" value="<?php echo $_SESSION['email']; ?>" /></td></tr>
<tr><td>phone</td><td><input type="text" name="mobile" value="<?php echo $user_mobile; ?>" /></td></tr>
<tr><td>productinfo</td><td><input type="text" name="productinfo" value="" /></td></tr>
<tr><td><input type="hidden" name="surl" value="success.php" size="64" /></td></tr>
<tr><td><input type="hidden" name="furl" value="fail.php" size="64" /></td></tr>
<tr><td><input type="submit" /></td><td><input type="reset" /></td></tr>
</form>
</table>
i have the following design for my website for customers to choose plans. when the user clicks the paynow button, they are redirected to this form.

when the use clicks the button,the corresponding amount should be added to the input field automatically. if its 400 the input should be 400 or 300 if 300. how can i do this inside submit button?