How to allow the user to enter the amount to pay

[netshine]

Assuming the name of your dropdown field is 'choose_product' (if not, amend the following code accordingly), try this:

$order = array();
$order['net_price'] = format_number(mosGetParam($_POST, 'Payment_Amount'), 2);
$order['tax_amount'] = 0;
$order['tax_rate'] = 0;
$order['product_name'] = "Project Payment";
$order['quantity'] = 1;
$order['product_id'] = intval(mosGetParam($_POST, 'choose_product'));
$orders[] = $order;