If you work it in reverse using the numbers you have posted, it makes no sense, no round number comes up.
I think 195 is likely the duty @15% (used the "other" category) because 195/0.15 = 1300 (your declared value is $13=1300 right ?)
Also, 306 divided by 0.17 gives a round figure of 1,800. And right next to 306 is written "ST" which could mean sales tax.
So this is a little confusing...you could have asked the post man for the meaning of each of these....
Ideally you should have simply paid it and gone to the post office for information for future reference.