We could go long into accounting and general concepts, and there may also be something you are supposed to declare in taxes if you are doing the law abiding citizen bit. However for low value items like this then it will probably be in line with base sales tax (and that may be an imagined value so you might have to go justify it with a bank statement/receipt), technically such a thing being called use tax in the US/Florida ( http://dor.myflorida.com/dor/taxes/consumer.html
) though it is probably the most evaded tax anywhere in the US.
Whether you get charged it... that would be a gamble really. Least likely is you wandering back through an airport with it, shipping is far more likely to see it pinged (though still not likely) and if you do not declare use tax yourself then for low value items like this whatever company is shipping it when stateside will usually get you to come to a depot to pay for it (or possibly pay online and have it shipped on). Depending upon what goes you might also be able to avoid having to pay European sales tax (usually around 20% but can be higher depending upon where you are buying it) which should offset 6% that quite nicely.