Learn English – “Make money off of” vs. “make money out of”

There is absolutely nothing wrong with

Since it's raining, wouldn't it be a good idea to bring an umbrella?

Look at a few other constructions to see how natural it is:

Since you're going to the store anyway, wouldn't you like to pick us up some ice cream?

Since our success is uncertain, wouldn't we be well advised to consider how to minimize our risks in case of a downturn?

The first clause assumes a fact, and the second balances it with a speculative counterpoint. That is grammatical, unexceptionable and, incidentally, one way to make a sentence interesting.

Only the phrase "I bought it online" is correct.

Saying that you bought if offline indeed makes no sense (unless, perhaps, you intended to make some sort of joke). "I bought it off online" doesn't make sense so avoid that one.

If you don't like the phrase "I bought it online" you could say "I bought it on the internet" or "I bought it from an online store". "I bought it off the internet" is also an option if you prefer to use the word "off".