Flat Rate Class Bug

Hi guys. I am having a problem with flat rate shipping. I have 5 shipping class on my store. Four of that classes needs to be able to send with flat rate. The other one is “Products above 75 pounds”. This class neeeds to be sended only by Correios. So, for each class i had to define a value, and for this class in particular, i leave it blank beliving that by leaving it blank woocommerce would ignore it and hide it from customers. But whe i was testing, the behavior wass otherwise. Woocommerce show it to customer and with no price at all. Is that a bug? Is this truthly the default behavior? How can i solve this problem? I hope that you could help me. Thanks