PHP is really flexible - you can include it in HTML pages or include HTML in PHP pages.
You will probably find you can keep most of your HTML pages and add php where you need it.
Think of it as PHP letting you do 'clever things' - most importantly pulling data from a database, as well as automating some actions (menus, rotating pictures, that sort of thing).
How much you will be able to customise the shopping cart system I don't know- Ive never played with them before
But, looking at some of the sample sites on the oscommerce website it would seem you can.
Its nice when hosts add features isn't it