If the store ordered 15 boxes, each having 80 pencils, they ordered 15*80 = <<15*80=1200>>1200 pencils.
If the cost of a pencil was $4, the store paid $4*1200 = $<<4*1200=4800>>4800 for all the pencils.
Twice as many pens as pencils that the store ordered are 2*1200= <<2*1200=2400>>2400
If A store ordered 300 more than twice as many pens as it did pencils, it ordered 2400+300= <<2400+300=2700>>2700 pencils.
Since the cost of the pens was $5, the store paid 2700*$5 = $<<2700*5=13500>>13500 for the pens.
The total amount of money they paid for the stationery is 13500+4800 = $<<13500+4800=18300>>18300
#### 18300