There are quite a few places where the author uses the expressions 'my heart sank', 'panic seized' etc. What was the reason for this?

The author has used the expression 'My heart sank' when the lady ordered for caviare. The author has used this expression to convey his inability to afford caviare. The use of this expression lends humour to the situation.

In yet another situation, the author uses the expression 'panic seized' to state that more than embarrassing it was a matter of concern when the lady ordered for asparagus as he anticipated that its price may exceed his budget line. The expression very aptly brings out the author's mental state of embarrassment and anticipation at that point of time.