Learn Finnish: Pakkaa uusi voileipä!

This is the main rule of the current lesson! The verb pakkaa is an imperative (a command). When you tell someone to do something using a command, the total object drops its usual genitive -n ending and stays in the basic nominative form.

Would the object change if this was a normal statement instead of a command?

Yes. If you were just stating a fact like 'I am packing a new sandwich', you would use the standard total object form with an -n ending on both words: Pakkaan uuden voileivän.

The dictionary form of the verb is pakata. Why does the command form pakkaa have a double k?

Pakata is a type 4 verb, which undergoes reverse consonant gradation. This means the dictionary form has the weak consonant (a single k) and the verb stem takes the strong form (a double k). The singular command form is simply that strong stem without any extra endings.

What makes up the word voileipä?

It is a compound word combining voi (butter) and leipä (bread). Even though it literally translates to butter-bread, it is the standard Finnish word for a sandwich.