Cars: optional extras or optional features

Most people would say features or something synonymous. After having said "optional," the word "extras" is somewhat redundant.

Is it possible to treat "optional" as a noun and call those car features just "optionals" (plural)?

Yeah, you can do that, and it would actually be indicating something a little different than just saying "options" would, I think.

For example, a choice between a blue vs black car would just be an "option" (with no extra cost).

And if you have to pay for an extra (optional), what do you call it?

Well, you could phrase it in many different ways. A common way might be something like "leather seats are optional," indicating that they are not standard, but you can get them if you're willing to pay more.

Some use the slang term "Bells and whistles"

"Optional extras" is a very common phrase in British English. It is pretty much the standard way of referring to things that a new car buyer might choose, such as satnav, parking sensors, bluetooth, adaptive cruise control, heated seats, etc.