As far as I am aware there is zero delay between you placing an order and a vendor receiving it. It is most likely due to the vendors being busy at that time dealing with older orders. For the larger vendors it is unlikely that they will have time to process a order, make up the pack and send it while dealing with other orders, all 45 minutes before they plan on leaving to ship.
If you absolutely must order an hour or two before a vendors cut off time I would try ThePopeOfDope (away for a week right now). Anytime I've ordered right up untill 12am he has been on the ball.
Or Radar breeder. Anytime I've placed a order before and sometimes after 2am it has been sent in the morning without fail.
I can't comment on anyone else since I've never ordered at cutoff. Although TGT could be worth a try (10AM cutoff).
I agree with you completely, a line is indeed a line. Cutoff times shouldn't be flexible, we depend on them being absolute.