Another Subjunctive Question
 

This is from the book I'm currently reading (translated from English to Spanish).

"Cuando Wense se acerca a la improvisada caseta de cobro, una destartalada camioneta pickup en la que el vidrio del ventana del lado del pasajero está roto, únicamente lo suficientemente para permitir que se lleve a cabo la transacción, regatea sin cesar pero en vano, con el vendedor."

My question has to do with the para permitir que part. I know that para que always triggers the subjunctive in adverbial clauses, but does that rule still apply when there is a verb (like permitir) in between the para and the que, and is that why llevar is in the present subjunctive?

Thanks in advance

While it's true that para que requires the subjunctive, having a verb in between changes things.
The reason the subjunctive was used is because permitir que always requires the subjunctive.

You could say:

Para permitir llevar a cabo
or
Para permitir que se lleve a cabo.

Or also it's acceptable to say. (Para que se pueda realizar la operación)

Pequeña corrección y yo usaría dependiente/cajero/a

Yes, cajero in this case: el cajero sólo cobra, el dependiente también atiende.