I haven't read the original Japanese edition or any other English translation. But, I'm sure Emma said "ハンカチを一枚" in the first page.
As you may already know, Japanese language doesn't have definite/indefinite articles such as "a, an, the". So if she said 一枚, she clarly indicated the number.
As for that "100, 200", I guess William said "100枚でも200枚でも", didn't he? If so, でも means the numbers are just an example or supposition. What William meant is "even though it is 100 or 200, that is fine."
In my opinion, William's answer is something like:
"A handkerchief? Fine! But Only one? I'll get you 100 or 200 if you wish."
The editor didn't have to emphasize the word "handkerchief" because William didn't care the handkerchief. He did care the price. He just wanted to impress it on her that he would buy anything, and really wanted to give her a more expensive present.