Wow, how much time have we all now spent discussing an expenditure of 47 cents?
First off, your personal taxes are your personal taxes. There is nothing business-related about them. The fact that they are "official" and "legal" does not make them business-related. If you get a speeding ticket on your way to the grocery store, that's "official" and "legal", but it has nothing to do with the company. I suppose your taxes are "business-related" in the sense that the income comes from the company, but by that reasoning, anything you spend money on is business-related. If you take your girlfriend to a movie, you're probably spending money you got from the company. That does not make it a business-related event.
As others have said, the simple answer is: Ask. In many companies, such minimal use of company resources is considered a perk of the job. Plenty of companies allow employees to use company phones to make personal phone calls, to use company-provided pens and paper to write a shopping list, etc, as long as the use is minimal and you don't take excessive time away from work. At other companies, even the smallest use of company resources for personal use is an offense that can get you fired. Big companies tend to have formal policies about such things printed in an employee manual. At smaller companies you just have to ask the boss what's acceptable.
If company policy says no, I can't imagine that it's worth arguing over the cost of a stamp. The only reason why this question is even worth spending five minutes discussing is that the issue could come up many times.