You've had really bad luck.
I think you actually get two messages after subscription - the first one after seconds confirming your order and the second one after a couple of minutes confirming activation of the ordered option. So don't start surfing before receiving the second one.
Recharging with € 30 would of course give you a buffer of max 41 MB before the Internet Flatrate XL is being activated, but you can save those € 10 if you're a bit more patient after subscription.
