Shouldn't it be: ... tamtę herbatę? It's in the Accusative case. I'm confused.
'Tamtę' doesn't exist. 'Tę', however, does.
So it's like:
Mam tę herbatę - I have this tea Mam tamtą herbatę - I have that tea.
No. Tę (ta) is just a specific word and it's the only adj in acc with -ę at the end. Every other adjectives in acc describing feminine nouns end with -ą.
No. Such usage of 'having' is equivalent to 'drinking', and that is not the meaning of the Polish sentence.
What about in a shop selling (dry) tea eg at the supermarket? How do you point out the tea you are 'having'?
No. Polish "tamten/tamta/tamto" is only English "that", while English "that" can be either Polish "ten/ta/to" or "tamten/tamta/tamto".
Read more here: "Why English "that" is sometimes Polish ten, sometimes tamten?"