When I pre-ordered High School DxD (Blu-ray) with Zavvi it didn't take the money from my PayPal until the release date...
BUT when I tried pre-ordering Fate/Zero part 1 (Blu-ray) after backing the Patema Inverted UE Zavvi "refused" to let me pre-order it due to not having enough money at the time of pre-ordering...
So... if you don't have enough money I believe you won't be able to submit your order. You could always try it.. But I believe it's one of those interesting flaws of PayPal.
Then again, it's better than some other sites that just take the money right away at the time of pre-ordering.
(And of course, this is all under the assumption nothing changed since I pre-ordered High School DxD earlier this year)