English Translation
“You’re lucky! Your change comes to exactly 256 yen.” “What do you mean by exactly?” “It’s exactly two to the power of eight.”
Very humorous sentence! I like it 
But the first two lines in English should be “I’m lucky! My change comes to exactly 256 yen”.
FYI: ぴったし is completely interchangeable with ぴったり. They can be spelled both in Hiragana and Katakana because they are onomatopes. The sound ピタっ illustrates something sticking to one another, like putting your palms together when you make a wish to Buddha. Here are some example sentences:
Wow, this gorgeous dress is the right fit for you! = うわぁ、この豪華なドレスは君にピッタリだね！
I arrived at his house at the exact time of the appointment. = 約束の時間ピッタリに私は彼の家に到着した。
The dialogue is ended with じゃん, which is transformed from ではありませんか？ (～, isn’t it?)–> じゃないですか？(more casual) → じゃん？(most casual)