A dollar is 100 cents, so standard postage is 108 cents.
Deborah’s four letters cost 108 * 4 = <<108*4=432>>432 cents for standard postage.
She paid 460 - 432 = <<460-432=28>>28 cents for international shipping on two letters.
Thus, the additional international shipping charge is 28 / 2 = <<28/2=14>>14 cents per letter.
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