Why not create a custom solution?

If they don't already have a managed database for their inventory that is kept up to date why not create one?

If the client has a POS system these typically use sql or access databases. If you can program or use visual studio you can easily create an ecommerce solution that will work magic for both ends.

He'll be able to track sales in store and online all up to date.

I have done work with OsCommerce (I have written a few contributions) and I have found that it is a great tool for beginners. In the long run however, it is easier to create a custom ecommerce solution. Using these 3rd party systems you'll run into issues with compatibility, functions that are needed or unnecessary, and your integrated payment system.

I have a few clients that HAVE to keep their inventory up to speed or it will affect their business terribly in the long run.

Originally the solution was the 3rd party software but you will quickly find a customized solution is the better one.

The best thing you can do now is sit down with your client and ask them what they need and want out of the system. If they're interested in an integrated POS system then offer it to them. If they're on a low budget go for a cost effective 3rd party solution.

Hope everything works out well for you

-Brando