so basically when you hit 901 real shipments, it becomes profitable to send 1000 - (number of real shipments) empty envelopes, at a total savings of $3 * (number of real shipments - 900).
(For number of real shipments <= 1000)
(For number of real shipments <= 1000)
0.9(3n + 1000 - n) < 3n
Which is the same as n > 750.
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.
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.