I think it is like any other delivery: they will tell you that they will do it between xx:xx and yy:yy. If you cannot make it part of that time, you should just call them right away and tell them that the time is not convenient and can they come later. If it takes you two minutes to get to the door, nobody will complain. If you ask them to wait 20 minutes while they are within a window which you did not say was inconvenient and therefore implicitly accepted, then they won't.

In London, many deliveries offer terrible time windows (e.g. 8am-8pm!!) I have tried to get greater specification many times but the answer is always the same - you accept the full window or you tell us that the window does not work for you and we'll propose another day when you can be at the delivery address from 8am to 8pm.