Learn Portuguese (Portugal): Ela mete o saco no carro.

Both meter and pôr mean "to put." However, meter specifically implies putting something inside or into something else, like putting a bag into a car or keys into a pocket. In Portugal, you will hear meter used very frequently in casual, everyday speech.

What kind of bag is a saco? Can I use it for a purse or a travel bag?

A saco usually refers to a shopping bag (plastic, paper, or cloth), a tote bag, or a sports bag/backpack. If she were putting a handbag or a travel suitcase into the car, you would use the word mala (or sometimes bolsa for a small purse) instead.

I remember that em means "in". Why does this sentence use no?

In Portuguese, the preposition em (in/on/at) must combine with the word "the" when they are next to each other. Because carro is a masculine word, "the car" is o carro. When you combine em + o, it shrinks into the single word no.

Could I say dentro do carro instead of just no carro?