Problems with URL search string

I have a form with a hidden field:

<input type="hidden" id="my_value" name="value" value="" />

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This field uses a value given by some javascript,  

document.getElementById('my_value').value = value;

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value is a variable which is given a numeric value depending on how the form is filled in. This appends 'value = ..' and whatever value 'value' holds at the time.

All the above works with no issue.

I am now trying to do the same again, but for a different form on a different page. I have my hidden field:  

<input type="hidden" id="my_fees" name="fees" value="" />

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I have my javascript:  

document.getElementById('my_fees').fees = fees;

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This does not work, the string always shows fees as 'fees=' and no value. Even if I give fees a value and remove any code that changes the value of fees in the script, so I have:  

function passenger_validation() {
	var fees = 699;

other code removed

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the string will always show 'fees='

Let me know if you need more code, or can spot some obvious flaw i've been blinded to in the second lot of code.

Alex