What happens if after the first block, this transaction become invalid? My best guess is that mybicoin will deduct this amount from my balance, and that is unacceptable for a shop.
Ok, you want to accept payment on 0/unconfirmed but you then want a guarantee against a double spend?
Unfortunately, the choice will boil down to either you accept the risk or you wait until one confirmation (or more, depending on the type of transaction).
Here is some more info: