this must have been written in relation to the last sentence

it is "jusqu'à là"
la does refer to a date which has been specified in the letter (ex: next holidays,summer)
depends on the context
jusqu'au la would have a musical meaning! la=A
hope it helps

Should really be written: "jusque là".

But this is something you would say in English (or maybe other languages too), not in French. In French, we would never end a letter this way. We might say "En attendant de se revoir" or "En attendant ce moment". But even this is not used very often.

In English, you can end a letter by saying "Till then" (refering to something you just said, such as for example "I am looking forward to seeing you again" or "We'll have a really nice vacation together", or whatever). You could also say "Until that time" or "Meanwhile", but "till then" is most often used.

This is obviously what the author meant to say in that awkward (awkward in French) expression.

Should really be written: "jusque là".

But this is something you would say in English (or maybe other languages too), not in French. In French, we would never end a letter this way. We might say "En attendant de se revoir" or "En attendant ce moment". But even this is not used very often.

In English, you can end a letter by saying "Till then" (refering to something you just said, such as for example "I am looking forward to seeing you again" or "We'll have a really nice vacation together", or whatever). You could also say "Until that time" or "Meanwhile", but "till then" is most often used.

This is obviously what the author meant to say in that awkward (awkward in French) expression.

P.S.: Of course, "meanwhile" (and even "until that time") would not be used by themselves. You would say "meanwhile, I remain your most devoted friend" or "meanwhile, I am sending you all my love" or whatever of that sort. With "Until that time" you usually add something too (until that time, all my love". "Till then" is often seen by itself, which is why I think it is a better translation.