# Set Valid/invalid flag to show/hide warning
@http.route(['/shop/cart'], type='http', auth="public", website=True)
def cart(self, **post):
    order = request.website.sale_get_order()
    
    valid = True
    if :
        valid = False

    values = {
        'website_sale_order': order,
        'compute_currency': compute_currency,
        'suggested_products': [],
        'valid':valid,
    }
    return request.render("website_sale.cart", values)
# fallback to "/shop/cart" if order not valid
@http.route(['/shop/checkout'], type='http', auth="public", website=True)
def checkout(self, **post):
      order = request.website.sale_get_order()
      if order.company_id.sale_freight_warning == 's' and len(set([line.product_id.has_frete for line in order.order_line if line.product_id])) > 1:
           return request.redirect("/shop/cart")
      else:
           return super(CorreiosL10nBrWebsiteSale, self).checkout(**post)

<!-- Add warning on website cart page -->
<template id="payment" inherit_id="website_sale.cart">
    <xpath expr="//h1" position="after">
        <t t-if="not valid">
            <div class="alert alert-danger">
                <strong>Wraning!</strong>
                You can't buy product with freight &amp; no freight together.
            </div>
        </t>
    </xpath>
</template>