[abbreviation] ID No., pursuant to Section 27a Sales Tax Act: DE ...

Note that legal system might get mixed up (Germany, UK, US). DE generally stands for Germany, and your query probably refers to German law.

UST is meant as an abbreviation for sales tax (or turnover tax)

This one is very tricky if you want to be precise. My response can give you just a rough idea.

Da es um Europarecht geht, würde ich VAT (value added tax) nehmen, nicht den amerikanischen Ausdruck.