Translation:One does not put ice cream into a package!
It becomes something like "One should(n't)".
Third-person imperatives also occur in sentences such as "God save the Queen" or "Long live the king".
You could also think about recipes; in German, they sometimes also use "one" as subject and then use a kind of imperative form, leading to things such as "One (should) take an egg and crack it into a bowl. Then one should add salt. One should put the mixture ..." etc.
In Esperanto, one could use "oni ...-u" for such things as well.
Or think of things such as "One should listen" - that could be "oni aŭskultu".
There are a number of prepositions that can take either the nominative or the accusative case: the nominative when they describe location and the accusative when they describe motion.
"En" is one of this group of adjectives.
"en pakaĵo" = in a package (location) vs. "en pakaĵon" = into a package (motion).
Other prepositions in this category include sur, super, sub, inter, trans, and tra.
(Notably not in this category is al, which always involves motion and so, since it doesn't have to make a distinction, follows the general rule that prepositions are followed by the nominative.)