From 26 U.S. Code § 4461: (https://www.law.cornell.edu/uscode/text/26/4461)
There is hereby imposed a [tax] on any [port] use.
(b)Amount of tax
The amount of the [tax] imposed by subsection (a) on any [port] use shall be an amount equal to 0.125 percent of the [value] of the [commercial cargo] involved.
©Liability and time of imposition of tax
(1)Liability The [tax] imposed by subsection (a) shall be paid by—
(A) in the case of cargo entering the [United States], the importer, or
(B) in any other case, the shipper.
(2) Time of imposition
Except as provided by regulations, the [tax] imposed by subsection (a) shall be imposed at the time of unloading.
It seems to apply to both imports and domestic cargo (exports are excluded). Is there some other law that exempts domestic port to domestic port cargo?