¿Cuál es la razón por la que envías estas flores?

That's a complicated way of asking "¿Por qué me envías estas flores'", hehe.

A nearly word by word translation could be done like this: "What is the reason because of which you sent me these flowers'", but of course, in English you'd say "why" instead of "because of which".

¿Cuál es la razón por la que envías estas flores?

Literal translation: What is the reason for which you send these flowers?

The way we say it: What is the reason that you send/are sending these flowers?

In Spanish we do things "for a reason," hence the "for which." The "la" just clarifies by helping the pronoun agree with its antecedent "razón" in number and gender.

I hope someone will correct me if I am wrong, but I think you could also correctly say:
¿Cuál es la razón por que envías estas flores? (no accent on que)

Oops, I didn't know Lazarus was around, or I wouldn't have jumped in. He doesn't have to think about his answers as long as I do.

Please don't appologize - it's a good thing to hear explainations from multiple people! It's just like having multiple reference materials. Thanks guys!