 Re: ..."as you go through"...
Two possibilities, which I think combine here :
1. If the continuous were used it would refer to the one moment of you personally passing through customs, whereas this is something that always happens to everyone. There's a sense of you being used impersonally here, so the simple form expresses the fact that is always true. Compare it with eg Keep stirring the mixture as you add the milk. In both this example and yours the continuous could be used, but it would give it a much more personal "one off" feel.
2. After a conjunction, the simple is always used used to refer to a future completed event. Examples :
When John arrives, call me.
As soon as the rain stops, we'll go out.
If you see Paul, let me know.
After the meeting finishes, we're going for a pizza.
As the course progresses, you'll find you understand more.
Unless you phone me, I'll expect you at six. |