There were few frills. Today's island stores are more modern, but don't expect scanners at every check-out just yet. What you will find is an expanded variety of international items in stock, meaning some familiar products among the Greek ones. Freshness is key and you won't be disappointed by fruits and vegetables. However, they may be dented or bruised just a bit from rough handling.
Costs are low, so go ahead.
On Greek islands, go for pottery, natural sponges, gold or silver jewelry and embroidered items. Local wines are also considered a good value. Smoking is usually allowed, even in food stores. You'll find sometimes even the person checking you out will be smoking. Some shops do exchange currencies, but you'll get a better rate at a bank.