Half of Bernie's collection means 18 * 0.5 = <<18*0.5=9>>9 postcards.
So he was able to sell 9 postcards for 9 * 15 = $<<9*15=135>>135.
He decided to spend all of his money on new postcards for $5 each, so he was able to buy 135 / 5 = 27 new postcards.
This means Bernie has 9 + 27 = <<9+27=36>>36 postcards after all.
#### 36