// add trade in battery to invoice
$('#btn-add-trade-in-to-invoice').click(function() {
    var bmfg = $("#invoice-trade-in-bmfg").val();
    var battery_size = $("#invoice-trade-in-battery-size").val();
    var tradeIn = $("#invoice-trade-in-type").val();
    var qty = $("#invoice-trade-in-quantity").val();

    // add to invoice array
    invoiceItems.push({
        battery_size: battery_size,
        quantity: qty,
        trade_in: tradeIn,
    });

    // regenerate the invoice
    generateInvoice();
});