The difference is that a postal service is rival + excludable. Most roads are (generally) non-rival and most people at least expect them to be non-excludable, but as you mentioned, some use tolls.

When you have rival + excludable, it is straight forward to charge on an individual basis. This efficiently allocates resources and punishes abuses.

It's not as easy to efficiently do that with non- (or perhaps less-) excludable goods. Also, when a good is non-rival, there is much less worry about abusing the system. That makes subsidies and "everybody pay the same" easier to be seen as a fair enough system.