Answers / better mailing list suggestions?

I need to track inventory, and can't think of the best method to do it.

I'm thinking that my best bet would be a separate "stock movement" table
that stores a positive or negative number for items coming in or out of
inventory for every order placed, or every shipment received. I could
relate each stock movement record to it's corresponding invoice as one
is created for each instance (sending or receiving).

There is fairly high volume though, and that means a SUM'ed query each
time I need to check if an item is in stock before placing an order
(telephone and web).

I thought about compiling the numbers into a total at the end of every
week or so, but this could quickly get complicated, and I don't want to
introduce any errors.

I need to be able to track stock movement history, and some
accountability would be nice (for example find a typo from a guy in the
receiving department, and make a stock adjustment, all with comments).

Any ideas?