Javascript sum currency amounts

I have the following routine that generates a "running total" of checkbox menu items (employee concessions), but it is running into two problems: (1) The first item checked does not appear unless another mouse-click is made on the screen, and (2) even though all amounts are currency, the total will occasionally display a bunch of decimal positions (e.g.,  10.59 + 2.25 displays as 12.8300000009).  
Here is the little function:

function checkTotal() {
	document.frmPost.runningTotal.value = '';
	var sum = 0;
	for (i=0;i<document.frmPost.Concession.length;i++) {
	  if (document.frmPost.Concession[i].checked) {
	  	sum = Number(Number(sum) + Number(document.frmPost.Concession[i].value));
	  }
	}
	document.frmPost.runningTotal.value = '$' + sum;
	}

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And the field that it is being displayed inside of is defined as:

<input name="runningTotal" type="text">

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Any insights would be very much appreciated!