As long as once you printed the receipt after buying the tax disc online, the DVLA should have this in your records that it has been ordered therefore it should be OK to use your car.
I had this before and my luck got pulled over but they were ok with the receipt.
i want to get my car tax, but post offices are closed an need to use the car tonight if poss, does anyone know if you pay for it online is it valid from that day or when you recieve your tax disk in the post?