This websro.correios.com.br link that I suggested is working for years now. AFAIK, they never changed the way it checks a package tracking number, so probably it won't give problems in the future. But if you don't wanna to replace one for the other, maybe if you add the one I suggested, so users could choose between Correios 1 and Correios 2, so if one stops working, the other may not.
Thanks for the answer!