Let's consider the following conditional sentence but without "ONLY".
ボタンを押せば、ジュースが出る。If you press the button, the juice comes out.
When "ONLY" is inserted with さえ, the conditional sentence becomes as follows.
A: ボタンを押しさえすれば、ジュースが出る。
Why do we not use B instead of A?
B: ボタンさえ押せば、ジュースが出る。
You can use both expressions fine.
Eg, Say you were explaining to a foreigner in Japan on using a vending machine who had never used one before. You could say something like this,
All you have to do is ,press the button and a drink will come out.
ボタンを押しさえすれば、ジュースが出る。
All you have to do is study and you will pass.
勉強さえすれば合格する。
I see where "only" is implied and using "all you have to do" as the meaning "you only have to do...."
Hope that helps.
Both A and B are correct. B's also not wrong at all.
