There is an implied difference between sending it to Amy and sending it on to Amy.
This is an instance where you need to use two words on to instead of the single word onto.
Onto Amy would imply on top of Amy as in: The dog jumped onto Amy. But in your example the underlying phase concerned is sending it on, which means forwarding it
So the difference between to and on to in this case is that one indicates merely that the item is going to Amy and the other indicates that it is being forwarded to Amy via the recipient from the original source. That's to say, it involves three parties rather than two.
For more explanations and examples of the differences between on to and onto, see: