Just send them the money. If the law doesn't let them send the part to you it isn't their fault, so they get to keep the money. Too bad!
The definition of a machinegun in federal law includes any part or combination of parts designed and intended for use in converting a weapon into a machinegun. In other words, that switch IS a machinegun, in and by itself, regardless of whether it is installed on a pistol. No way is it "legal" to buy, import or own today.
Jim
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