In the two examples you cited, Papier and Kaffee are uncountable nouns, so when there are quantitative modifiers, they remain singular; If it is a countable noun, when the preceding quantity >; 1, it becomes plural;

The singular and plural changes of nouns that constitute the concept of quantifiers follow the following rules:

A) When masculine or neuter nouns form quantifiers, there will be no plural changes:

A cup of coffee, a cup of coffee

B) When the feminine nouns form quantifiers, there will be plural changes:

A piece of black paper, zwei Bl? Tter paper

A cup of coffee, a cup of coffee

I wish you a happy study!