Learn Finnish: Lähetämme palautuksen isommassa paketissa maitse. - We send the return in a larger package by land.

The prolative case ending -tse is almost always added to the plural stem of a word. For maa (land), the plural stem is mai-, so adding -tse gives us maitse. It translates to 'by land' or 'overland', indicating the route or medium of transport.

Could I just use the adessive case and say maalla instead of maitse to mean 'by land'?

No, maalla means 'in the country' (as opposed to the city) or 'on land' (as opposed to in the water). To express the route, pathway, or method of transport through a medium, you need the prolative case maitse. Other common examples are meritse (by sea) and postitse (by mail).

Why is the object palautuksen in the genitive-like accusative case rather than the partitive?

The action of sending this specific return is viewed as a complete, bound action. Using the accusative palautuksen shows that the entire return will be sent as a finished deed. If you used the partitive palautusta, it would imply an ongoing, unfinished process ('we are currently in the process of sending').

Why does the comparative adjective isommassa have a double m instead of mp?