Learn Finnish: Lähetimme seurantakoodin sähköpostitse.

The suffix -tse expresses the route or medium used, translating here as "via" or "by." While the prolative is a rare case overall, it is very common with means of communication (like puhelimitse for "by phone"). You could absolutely say sähköpostilla (using the adessive case) in everyday speech, but sähköpostitse is slightly more formal and very typical in business or customer service contexts like this one.

Why does seurantakoodin end in -n instead of being in the partitive case?

It is the total object of the sentence. The action of sending is complete, and a single, specific tracking code was sent in its entirety. Because the verb lähetimme is a normal finite verb with a personal ending, the total object takes the genitive-like -n ending.

Where does the seuranta part of seurantakoodin come from?

It comes from the verb seurata, which means "to follow" or "to track." Adding the noun-forming suffix -nta turns it into seuranta (tracking or monitoring). Combined with koodi, it forms the compound word for "tracking code."

The dictionary form of the verb is lähettää. Why does lähetimme only have one t?