Translation:I will collect your mail until you come back.
collect = sammeln
collect mail = post abholen
post sammeln is unidiomatic.
I also don't get why zurückkommst is all one word. Would it be incorrect if it was 2 words?
Because zurückkommen is a separable verb. Since bis here is a subordinate conjunction. It pushes the conjugated verb to the end. Now, that it's a separatable verb, the prefix goes between bis kommst. When this happens, it's reattached to the verb, hence zurückkommt. You have to do this.
See how in this sentence bis is done? „Kannst du nicht warten, bis wir an der Grenze sind?“
In this sentence totlachen is tot|lachen. „Der Igel war tief verletzt, weil der Hase sich fast totgelacht hat.“
That doesn't make sense in English. 'Posts' are usually large, straight pieces of wood, whereas this sentence is talking about mail.
'Post' is also synonymous to 'mail'. Though nowadays it refers to a piece of writing published online -- example: Facebook Post.
I'm aware that 'post' in the singular is synonymous with 'mail', hence my only mentioning the plural- your original question.
When using 'post' to refer to mail, it's an uncountable noun- this means that it cannot be pluralised to 'posts'.
Also, the German sentence isn't plural, so it can't be translated with a plural.