Should it accept "post" instead of "send"? Most people in the UK would say "post a letter" rather than send...
Could we say "Ili volas sendi leteron al vi"?
Isn't it rather "a letter to you"?
Your comment would seem to make more grammatical sense, but the original phrasing of words could also work.
So, "ili volas sendi al via leteron" would be sending you your own letter, right?
Ili volas sendi al vi vian leteron