Cualquier v. Cualquiera v. Cualesquiera

Question

I am sure that I am missing something that is right before my eyes, but I am going to ask for clarification anyway.

The lesson states that "cualquier is used BEFORE a singular noun."

Further it indicates Cualquiera must be preceded by un or una + and comes AFTER a singlular noun.

Lastly it references the plural of Cualqiera: cualesquiera. This has no requisite precedent (un/una).

Therefore, please explain if there is an exception as apparent in the following:

"La Unión estará de CUALESQUIERA derechos de aduana." This test question has no article precedent (un/una) and CUALESQUIERA appears BEFORE the noun (derechos de aduana); that is, it appears to act more in line with the singular CUALQUIER. So the question is: Is CUALESQUIERA the plural of Cualquier and not the plural of Cualquiera. Thank in advance for your help and response.

Accepted Answer

Hola Armando
The plural "cualesquiera" is the plural for both cualquier and cualquiera, it doesn't mind if you place it before or after the noun. (The form cualesquier used to exist but not anymore) 
You don't need un/una because it is the plural, not the singular, so in your sentence (which was missing an important bit):
La Unión estará exenta de cualesquiera derechos de aduana.
The Union will be exempt from any customs duties.
You have the plural cualesquiera in front of the plural noun "derechos".
I hope this clarified it.
Inma