He uses 1 packet per cup of coffee and he has 2 cups a day so that's 1*2 = <<1*2=2>>2 packets
He wants enough to last him 90 days and he uses 2 packets a day so that's 90*2 = <<90*2=180>>180 packets
The packets come 30 to a box and he needs 180 packets so that's 180/30 = <<180/30=6>>6 boxes of sugar substitute
Each box costs $4.00 and he needs 6 boxes so that's 4*6 = $<<4*6=24.00>>24.00
#### 24