The idea that sending out blank envelopes so that you can reach the threshold for discount is genius, never thought of that. Might have to look into it for my business.
Now a shipment usually costs me around $3 to mail. At that 1000 threshold the discount will suddenly go from $0 to $300. Sending an empty envelope would cost about $1.
(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.