[cake]

Shit I just spent an hour researching this !

Something was fishy in your inequation here is my result :

3x > 3x+(1000-x-300)

So this means that at x > 700 it is profitable to send empty enveloppes.

[zerohp]

I think your equation is modeling an exact savings of 300 for sending 1000 pieces of mail. The discount is 10% for sending 1000 pieces.

Mine is simply comparing the cost of sending n pieces of mail at $3 each to 90% of the sum of n pieces at $3 each and 1000-n at $1 each.

[cake]

Yes I assumed a 300 flat rate from the previous comment, but you're right he mentions 10% in his post.