why is the subjunctive not used in the example - Les fleurs que Paul sent?

Stephen is correct. "Que" in this sentence is a relative pronoun not a subjunction. To tell the difference, ask yourself the question if "que" refers to something in the sentence. In the case "Les fleurs que Paul sent", it refers to "les fleurs". In these cases it doesn't mandate the subjunctive (with some very specific exceptions).

Even if "que" is used as a subjunction it doesn't necessarily and always require the subjunctive (otherwise French would be too easy ;)). For example:

1) Je vois la valise que tu as apportée. -- "Que" is a relative pronoun referring to "la valise".
2) Je sais que tu seras en retard. -- "Que" is a subjunction but no subjunctive required.
3) Je doute qu'il soit en retard. -- "Que" is again a subjunction but in this case the subjunctive is correct.

-- Chris (not a native speaker).

I think because the 'que' refers to the flowers...the noun... and not the rest of the sentence/the following phrase...but I stand to be corrected )))