lol - well, here's a trick:
   When you put an item in your basket, notice how much it is.

   Mentally round it up to the nearest 5 and then forget about.

   So if it's 2.99, make it 5.
   if it's 8.99 make it 10.

   stuff like that.

   And you can round some things down if they're really close but
   rounding up is better.

   Then by the time you get to the checkout line it usually works
   out to "close enough" and make up for the mistakes and the
   supermarket games they play, where you THINK something is one
   price but it's really another price.... or where they have "buy
   2 get 1 50% off" - shit that few people can calculate directly:
   they know that all ppl are going to think is, "Oh, it must be a
   good deal"... even when it's not. Been doing it for so long, I
   can just look at anybody's pile of groceries and have a number
   pop up in my head. Might just be some weird way I'm wired too.