The paper we use is white 100gsm premium treated stock and we use generic white envelopes. 

For postcards we use 350gsm card plus lamination and for mono postcards we use 250gsm card without lamination. 

Mailings up to 14 sheets of paper will be in "C5" envelope (A4 folded in half), and mailings of 15 or more sheets will be in "C4" envelope (A4 unfolded).  

Our Business reply envelopes are DL size.