Infix, prefix and postfix are merely notations. For example
1 + 2 : Infix, The '+' operator is written between the operands it applies to.
1 2 + : Postfix (Reverse Polish Notation), the '+' operator follows it's two operands.
+ 1 2 : Prefix (Polish Notation), the '+' operator preceeds it's operands.
Precedence rules ("order of operation") have nothing to do with ASCII values, as mentioned. These are just rules made to clarify unambiguously which procedures should be performed first in a given mathematical expression.