Is Taiwan R. O. China?

Today I went to the USPS to mail a letter to Taipei. I asked A's advice on how to write the address properly since he grew up there. So I wrote something like this on the envelope:

XXXXX
XXXXX (in Chinese)
Taipei, Taiwan
R. O. China

The clerk took my letter and started key in destination on her computer. I happened to glance at the screen on my side and noticed "China". So I pointed out to her that it should go to Taiwan, not just China (the Mainland). Then she said, "You should write down 'Taiwan' at the bottom, then. Otherwise it won't go there." I said, "I have 'R. O. China' at the bottom. Isn't that good enough?" She said no.

So apparently according to at least one employee at the US Postal Service, R. O. China doesn't equal to or contain Taiwan. Then I wonder, whether this means that they believe there is only ONE China, or that Taiwan is NOT China.

Now that is really puzzling!