Learn Finnish: Minä olen tänään kaupassa.

This is because of consonant gradation. When you add the "in" ending -ssa, it closes the final syllable of the word. For many Finnish words, this causes a strong double consonant like "pp" to weaken into a single "p". So, kauppa becomes kaupassa.

Why is the ending -ssa and not -ssä?

This is due to vowel harmony. The root word kauppa contains the back vowels "a" and "u". Because of this, any endings attached to it must also use back vowels, which is why we use -ssa instead of -ssä.

Could I change the word order to Minä olen kaupassa tänään?

Yes, you can! Finnish word order is quite flexible. However, it is very common to put time words like tänään (today) right after the verb, which is why it comes before kaupassa here.

Can I leave out the word minä and just say Olen tänään kaupassa?

Yes, that is perfectly natural. The verb olen already has the -n ending, which clearly tells us it means "I am." Because the verb leaves no doubt about who is acting, the pronoun minä is optional and often dropped in everyday speech.