Unambiguity of Reverse Polish Notation
To show that a grammar is unambiguous, it is enough to show that for any expression E, there is only one "last step" possible in any derivation of E. It is the case here : the last rule is given by the last symbol of the expression (either +, *, or a terminal a), and the parentheses will prevent any ambiguity.
Of course you can not write "$abc+$" in your grammar, it has to be $(ab)c+$ or $a(bc)+$, but this is implicit when you define a grammar.

For instance, $a(bc+)*(bc)*+$ is not ambiguous : the last rule is given by the last symbol +, and so on... the expression it represents is $(a*(b+c))+(b*c)$