Lähetys oli pieni, mutta sen hakeminen pakettiautomaatista kesti pitkään.

Lähetys literally means shipment, delivery, or consignment. In context, it can refer to the item that was sent, so English may translate it as package.

It is close to paketti, but not exactly the same in nuance. Paketti is the physical parcel itself. Lähetys can sound a little more like shipping/logistics language.

After olla when you describe what a singular countable thing is like, the predicate adjective is normally in the nominative.

So:

lähetys + oli pieni = the shipment was small

Here pieni matches the basic dictionary form of the adjective.
Pientä would be partitive, which is not the normal choice here.

Both oli and kesti are in the Finnish imperfect, which is the normal past tense for completed past events.

So the sentence is describing one past situation:

• Lähetys oli pieni = the shipment was small
• sen hakeminen ... kesti pitkään = picking it up took a long time

This matches English simple past very closely.

Hakeminen is a -minen form made from the verb hakea, to fetch / to pick up.

This form turns the action into a noun-like word:

• hakea = to pick up
• hakeminen = picking up / the act of picking up

So it behaves like a noun in the sentence, even though it still keeps a verbal meaning.

It comes from the verb stem of hakea plus -minen.

Very roughly:

hakea → hake- → hakeminen

You do not keep the final -a of the basic verb form.
This is a very common pattern in Finnish:

• lukea → lukeminen
• tehdä → tekeminen
• syödä → syöminen

Sen refers back to lähetys.

So sen hakeminen means picking it up or more literally the picking up of it.

English speakers often first read sen as if it meant its, and structurally that is not a bad way to think about it. But in practice, this whole phrase simply means fetching it.

With -minen nouns, Finnish can still show the object of the action, and here that object is sen.

Here the parcel is understood as a definite whole object that gets fully picked up, so Finnish uses the total-object form.

For se, that form is sen.

So:

• sen hakeminen = picking it up

Using sitä here would not fit the intended meaning well. It would suggest a partitive object, which is not what this sentence wants.

Pakettiautomaatista is in the elative case, ending in -sta.

The elative often means out of or from inside.

That fits the situation well: you are taking the parcel out of the parcel locker.

So:

• pakettiautomaatista = from the parcel locker

If you said pakettiautomaatilla, that would mean more like at the parcel locker, focusing on location rather than removal from it.

The whole phrase sen hakeminen pakettiautomaatista is the subject of the second clause.

The head word is hakeminen.

So the structure is basically:

[sen hakeminen pakettiautomaatista] kesti pitkään
= [picking it up from the parcel locker] took a long time

Pitkään is an adverb meaning for a long time.

Pitkä is an adjective, so it describes nouns:

• pitkä päivä = a long day

But here we need something that describes how long the action lasted, so Finnish uses the adverbial expression pitkään:

• kesti pitkään = lasted a long time / took a long time

This is the normal idiomatic choice.

This is a very natural, neutral word order.

It goes in a clear order:

• subject: sen hakeminen pakettiautomaatista
• verb: kesti
• time expression: pitkään

Finnish word order is flexible, but changing it changes emphasis. Here the sentence is just presenting the information plainly.

Yes, but the structure would have to change.

For example, you could say something like:

Lähetys oli pieni, mutta kun hain sen pakettiautomaatista, siihen kului paljon aikaa.

That means roughly the same thing.

The version with hakeminen is useful because Finnish often likes to turn an action into a noun-like idea: the act of picking it up.