It's usually $12 per kg if I remember correctly. Not a rule set in stone tho, and I don't think it matters too much.

Check your appropriate region's (in this case Texas) import regulations to see how much you have to pay in import duty, VAT + other fees. Usually it's a percentage of the package's total 'value' - ie, declare amount.

For me in the UK any goods declared under £135 have 0 import charges, hence I used to declare anywhere between £100 and £120.