There is no general price compiling website for shops in Peru far as I know. (Maybe a market opportunity for someone?) But you certainly can get prices and buy online for certain shops (Sodimac, Maestro, Ripley, Saga, Wong, etc.)
Our general strategy is:
Weekly grocery shopping: Plaza Vea (cheaper than Wong, better fresh food than Metro.)
For meat we go to a local market, e.g. Surquillo, buy about a month's worth and freeze it.
For bottled water, we buy about 150 litres worth from Macro every 2-3 months.
Children's toys for birthdays etc. (a significant expenditure when you have kids with parties every few weeks) we buy opportunistically whenever there's a promotion in Tai Loi, Wong etc.
For car parts and accessories, there is no single go-to shop for everything (another market opportunity maybe?) and generally you have to shop around depending on what you need. Never bought car tyres, but I know there are some online stores, e.g. http://ekonollantas.com/
and they should be easy to compare. (Google llantas Lima)
Hope that's of help.