Plural objects follow the same logic — whole or partial:

Total plural: -t	Partial plural: -ja / -jä, -ita / -itä
Ostan munat. — I'll buy the eggs (all the ones we need).	Ostan munia. — I'll buy some eggs.
Syömme mansikat. — We'll eat (up) the strawberries.	Syömme mansikoita. — We're eating strawberries.

• The total plural object looks just like the ordinary plural, ending in -t: munat, mansikat, kupit.
• The partial plural is the plural partitive you met at the market in Unit 4: munia, marjoja, mansikoita.

One surprise: jauhot (flour) is a plural word in Finnish, a bit like English scissors. So: Tarvitsemme jauhoja. — We need (some) flour. Ostan jauhot. — I'll buy the flour.

And where do you buy them? Finnish buys things from a shop, not in it: Ostan munat kaupasta. — I'll buy the eggs at the shop.