My boss says "Find coverage or you cannot call out."?

That depends on where you are. If your boss knows, then I assume you've already said you are calling out.

Why not take an Uber? It'll save you time and a 5 mile Uber probably costs like $7

Yes, she can legally fire you for no reason. If you have a union that negotiated that sick time for you, then the union can defend you. Without one, the employer can dictate how and when you use sick time. Most employers require medical documentation. If your appointment is outside of work hours, and it is, you have no medical documentation. It sucks to have an employer who doesn't care about your outside life, but that's a reality of life. There aren't (and should not be) laws that dictate everything.

The juvenile chat session seems to indicate you all do not know what the hell you are talking about because the boss woman can fire you for this or any other reason she so desires without ever even discussing it with you. So all that so called documenting will get you no where as far as keeping your job.

Is the funeral immediate family? Wake, nay however if you are really ill then you need to keep the Doctor's appt' and get to work

There is no law that says 2 people have to be working at a gas station, that's just company policy or something the boss made up.

Yes, she can legally tell you that your fired if you call out. And you can legally tell her to go ahead. It's a gas station. There's a lot of employee turnover at those places and you can find another job easy if she doesn't back down.

Unless you have a union, your employer is not obligated to obey any normal customs regarding funerals, etc... Going to a wake isn't a religious ritual. You can't claim a right to take a day off for a funeral.

Even your medical excuse doesn't pass muster as you're largely claiming you need the day off, because you don't have transportation. They're required to accommodate medical problems, not to allow you to make late afternoon appointments and to get their as slow as possible.