If a product is supplied free-of-charge on the proviso that it appears in-vision (or might appear in-vision) that would be considered dodgy if you were avoiding product placement in many situations. (Not all - but many)
Out of curiosity, how would they usually go about stocking a shop such as the corner shop in Corrie? Would the props buyers just go to Booker* and buy a shops worth of items? Do the cans of coke in the fridge get replaced or do they just sit there having gone out of date?
* Other wholesalers are available.
Yes, they will buy what they need to buy, or rent/borrow other items. For long term programmes/soaps they'll have a larger budget for procuring things.
Perishable items will be removed from packaging and refilled with something non-perishable that will happily sit there for years. Bakery items, like "bloomer" loafs can be varnished with many layers of polyurethane, and will remain delicious looking for months (I did a few of those techniques in college).
Stuff that's in cans will certainly go out of date, but its not unheard of to use upside-down empty tin cans with colour copied labels pasted on to them - and et voilą - you've got a supermarket style selection of soups and beans and pet food - this is important if you've got shelves attached to scenery flats which don't support much, if any, weight. Plastic milk bottles can be empty and internally coated with white paint.
Really depends on the show, the budget and the location you're filming in.
High Road used to swing the fourth wall of the shop set closed and padlock it to stop things being nabbed off it. Studio A was open and easy to wander into