What to buy:
Alcohol is very cheap in Andorra as its a duty free country. Beware though that there are restrictions on how much alcohol you can take with you back across the Andorran boarder.
What to pay: Its Euro2.40 for a litre bottle of vodka, maybe Euro10 for the better known brands.
Pic Negre are a chain of ski / snowboarding equipment stores. You can hire all your gear for the slopes from them, skiis, ski boats and poles.
It is usually packed on Monday mornings because everyone goes here to hear their equipment for the week. Expect long queues.
While the restaurants at the piste are reasonably priced- Water and chocolate cost 3 times as much above the cable car as they do below the cable car.
As to other shopping- get real this is a tiny ski resort- they don't do shopping!!
You can go to Andorra La whatever, to avail of "tax free" shopping, but you save sod all, and loose a day's ski-ing which you have already paid for!- pointless methinks (but I'm not much of a shopper anyway...
What to buy: Mars Bars and Skis
Apart from souvenir t-shirts, there is practically nothing in terms of clothes shopping in Arinsal. Most of the shops there are geared to tourists like the ski-hire shops, outdoor adventure shops. Theres a pharmacy and a supermarket.
What to buy: Any extra ski-stuff you require.
But apart from that, you're better off just going to Andorra-la-Vella for anything and everything else