The times that the shop restocks are:
12.00am 3.00pm 7.00pm
If you want to get a quick restock in your shop, set the time to 11.59pm, 2.59pm or 6.59pm, wait one minute and the shop should then be restocked. At the midnight restock your tama will obviously be asleep, so its probably easier to just reset to 2.59pm or 6.59pm.
//Haze