A coloured envelope, a gift box,... It doesn't matter, you need to be sure your important mail doesn't get lost. Print the sender address on a mini label and put that on the parcel or backside of the envelope: it will help the postal office to return the undelivered mail. Furthermore a letter can, between tons of other letters, be the first impression you give a company. So you need to be sure your letter looks professional. A transparent mini label is the best solution, especially on a coloured background it gives a qualitative impression!