The official currency is the euro €. Swedish krona SEK is usually accepted in most shops and restaurants during the peak season, but the exchange rate is typically not very good, so it is typically significantly cheaper to pay in euros or by card. Compare rates with local banks first.
Please note: Shopping in Åland is very expensive. Due to import of most goods, with sometimes unsurmountable difficulties in filling the stores with enough supplies, prices in most stores are in the EU highs, mostly 10-50 percent higher than in the Stockholm or Helsinki metropolitan areas. Prices on some groceries can be even higher, with oatmeal and gruel selling at more than double the price in, say, Stockholm or Helsinki.
SALT, in the maritime district of the east harbor, Mariehamn has a selection of local artisan crafts including textiles and glassware, made in Åland. Next to a maritime museum and local jewellery shop.
Åland has its own postal service and stamps unique to the island, these make excellent inexpensive souvenirs. Post offices are in several towns and in Mariehamn.
In general, souvenir shopping in Åland is difficult. The island is not well presented for international tourists and there are only small displays of the typical souvenirs such as keychains, postcards and mugs. A few of the stores along Torggatan in downtown Mariehamn have these, as well as museums and some of the larger grocery stores.
Local beer and apple liquors are tasty and available.