Is it possible to have a line item that is percentage based of the other lines on the sales order/invoices?

This wouldn't be a tax but a supplies charge that is based off the price of the other items being sold.

I've tried making a field in studio and making an automation but it doesn't seem to be working.

# Check if 'x_studio_shop_supplies' exists on any of the order lines
lines_with_supplies = record.order_line.filtered(lambda line: line.x_studio_shop_supplies > 0)

if lines_with_supplies:
    # Get the percentage value from the first line with 'x_studio_shop_supplies'
    percentage = lines_with_supplies[0].x_studio_shop_supplies / 100

    # Calculate the sales order total (excluding taxes and any existing "Shop Supplies" lines)
    total = sum(line.price_subtotal for line in record.order_line if line.product_id.name != 'Shop Supplies')

    # Calculate the shop supplies fee based on the percentage
    shop_supplies_fee = total * percentage

    # Search for the "Shop Supplies" product
    fee_product = env['product.product'].search([('name', '=', 'Shop Supplies')], limit=1)

    if fee_product:
        # Check if a "Shop Supplies" line already exists
        existing_fee_line = record.order_line.filtered(lambda line: line.product_id == fee_product)

        if existing_fee_line:
            # Update the existing "Shop Supplies" line
            existing_fee_line.write({
                'price_unit': shop_supplies_fee
            })
        else:
            # Add a new line using the Command namespace
            record.write({
                'order_line': [
                    Command.create({
                        'product_id': fee_product.id,
                        'product_uom_qty': 1,
                        'price_unit': shop_supplies_fee,
                    })
                ]
            })
else:
    raise UserError("No order line with 'x_studio_shop_supplies' found.")