Do the latest pocket pc's still come with Pocket Excel?
I would just make a spreadsheet in that with all the stores,dates, items, and prices.
You could make seperate ones for groceries, toys, etc.
You didn't specify what OS and CPU your pocket pc has. (mine has WinCE 3.0, pocket pc 2000, SH3 cpu, as an example) What would be a good solution might run on one pocket pc but not another.