Ha! I had to close the shop because in earlier testing people were spending all of their money the last second...
Also a tip about the +10 seconds is that you only want to buy it when you know you can make the price back within that time:
EG: if the price is only 60, make sure you get minimum of 60 points in 10 seconds after buying it
This is also true for the idle scores like you never want to get idle scores when the time left is less than the score:
EG: if the price is 60, only buy +1 idle when there is more than one minute left, else you'll end up with a loss :P