The plugin will need to round the number to 2 decimal places (it is the only accepted format for the payment gateway.
When your price is $5, the tax amount on a 5.5% rate come out to be 0.275. When that amount is rounded to 2 decimal places, it will be 0.28.
So as far as I can see, the tax amount looks correct to me (I have just checked the two items you have on the page that you shared).