Surprise surprise! Yes, you can get some souvenir at the Cafe. They sell t-shirts, key chians, snacks, and etc.
Just want to point out that climbers can buy postcards from the counter and they can mail it for you at the cost of RM5 for international mailing.
What to buy: The most special is to buy a postcard and mail it from here.
What to pay: RM5 for international mailing
On the way from Kota Kinabalu to the Kinabalu National Park, you cannot miss a parking lot full of small souvenir shops. From here you have a great view of the mountain if the weather is good, and it is a great place to do some souvenir shopping.
The great thing about this place is that there are plenty of little shops to look around in and that the salesmen are not annoying and just let you have a look. The prices don't really differ between the different shops, but some shops do have different things for sale. Prices are all rediculously cheap, so you don't really need to bargain.
What to buy: You can find lots of different souvenirs here: from hand-made instruments to clothes and from statues to kitchen-gear. I can't imagine you won't be able to find a nice souvenir here.