They are actually related with こそあど言葉, shortly saying こ of これはこれで means this, in English, same of そ of それはそれで means that, in English.
Granted, kindly take a look at this picture. You can find the relationship of こ/れ，そ/れ．
So your first sentence, the speaker is 俺｛おれ｝, in another words, I, in English.
Pardon me if my translation is not good enough,
・・・I prefered the letter be inside ( my ) shoe's box. But I can understand this ( kind of style ) shows ( she ) has a crush very positively on me.
So, here, the speaker, I is talking about the letter which the speaker - I - have received. As you can see from the picture I presented above, you can see the letter is just beside the speaker.
And you second sentence.
( I am sorry since there is no prior or latter sentence with this, I guess personally.
・・・Alright, that is understandable, but even though I know the right answer, there is a problem remaining how I can master that 「sprachgefühl」
So the speaker, here ( and since it seems the latter sentence has nothing to do with the first sentence ) , the speaker is speaking or pointing about which someone else ( but related with the speaker ) said or someone else has given.
So as you might be able to guess from the picture, これはこれで, if I were you, I would translated,
Taking this for granted,
whereas それはそれで、same according to the picure,
Putting that aside for a while,