[dmlorenzetti]

A few minor UI comments.

+ Failing to select an amount to send should raise a flag.

+ Entering a negative amount to send should raise a flag.

+ After viewing a result, if I hit `back` on my browser (say, to compare results for a different transfer amount), it resets the currencies to their defaults (for me, U.S. and Argentina). It would be nicer to remember my selections.

[Rulero]

Thank you very much for your feedback, will get these changes implemented :)