Hi there, if I remember well it is grammatically incorrect to use "will" with "when"... is that true? or sometimes there are some exceptions...
"When will you send it?" is a mistake or not?
It's not. It's perfectly OK.
I think the rule you're misremembering is about the tense in a time clause that refers to a future event: "When I arrive, he will be there" - in that case you wouldn't use a future tense in the 'when' clause.