IF you buy shelves at big box stores, make sure that the shelves will do what you want.
I recently bought 10 sets of 2x4 shelves at Lowe's with particle board shelves that sit on a steel perimeter lip about 1/4". Those shelves sagged within days of installation and very light loading (shelves box says 200lbs per shelf) perhaps 40-50 lbs per shelf. With only three of the five shelves in use.
I contacted the maker and they were kind enough to send out two braces to span the 2' sides and add support to the structure. When I told her I needed 80 to do my shelves, like she suggests, she said she could send 4 pair of braces? Called Lowes and they got me 30 braces and suggested one brace per shelf.
Almost $100 per unit and I have about 30% capacity on half my shelves. The others are unusable and zero support from the maker or box store.