Shopping opportunities in Lithuania are not of inferior quality than in the West and generally superior to the countries to Lithuania's east.
Shopping mall is a key institution in every Lithuanian city. Hundreds of shops within each mall sell all types of goods while eating out and entertainment opportunities are equally wide. It is common to spend an evening or Saturday at the malls which are typically gazetted as "shopping and entertainment centers". Many malls are centrally located and usually larger than one would expect from city size.
This "mall craze" started with Akropolis (Vilnius) in 2000, which by then was likely the largest in Eastern Europe. Now each of the 4 main cities has its own Akropolis mall but the brand has since been joined by multiple local competitors, among which Ozas (Vilnius) and Mega (Kaunas) are the largest.
Lithuania also has a wide network of quality supermarkets, the larger of which also sell nearly everything under a single roof, even if the variety of goods is smaller than in shopping mall.
Shopping malls and supermarkets have very convenient opening times, working 7 days a week from early morning until some 22:00 or 23:00 in the evening. As such, there are no convenience stores in Lithuania. They are in part replaced by gas stations stores which stay open 24/7 (in the main cities and on the main roads) and charge ~50% inflated prices on drinks and food. Alcohol sales are forbidden at night and at September 1st.
The advance of shopping malls heavily hit the traditional city high streets, especially so in the smaller cities. Fewer shops and services are available there. Still however they are far from empty despite what a small business lobbyist would tell you. As a tourist, it might be more interesting to stroll around high street than a shopping center.
Archaic mostly-outdoor bazaars remain another popular way to shop. Small businessmen sell everything from berries and pirated CDs to wedding dresses and furniture there from their little shops, kiosks or even car trunks. It can be cheap, but one can also get overcharged - so shop around and haggle. Joining the crowds of Lithuanians shopping in the enormous "Wild East" bazaars like Gariūnai even during storms is an experience on its own.
The bazaars have now been joined by new Western-style marketplaces that are mostly aimed at people who prefer organic food. Prices in such markets might be higher than in bazaars.
If you want to buy souvenirs you can buy them both in the streets frequented by tourists (main cities and resorts) or in the supermarkets. The supermarkets mainly offer mass-produced Lithuania-themed Chinese-made t-shirts, cups and fridge magnets. The street stalls additionally have a selection of traditional crafts, amber jewelry, and relatively cheap paintings. Haggle if you buy at street stalls and be prepared to severely overpay if you buy in downtown shops established for tourists.
Fairs are another good option for souvenirs. Typically coinciding with local religious and secular holidays they draw in many salesmen from surrounding cities and towns (including craftsmen) who transform the downtowns into large marketplaces for a weekend.
Haggling is possible only in bazaars, marketplaces, souvenir stands, and fairs. The price may be lowered by 10%-30% (even when official rate is written). Foreigners may, however, be quoted a higher initial price, therefore check multiple salesmen before buying.