Ummm... "Tripped" I think: Knicknacks and Whotnots and Bits and Bobs
Nice little shop selling all the things listed in the title. I didn't get distracted though - as I was on a mission to get to the Royal Botanical Gardens.
What to buy: I did have a quick look at the postcards - but bought nothing .
What to pay: 50p for postcards - dunno how much for the hammock.
The souvenir shop: Seeds and organic sweets
What could a souvenir shop in a place like Kew Gardens sell? Flowers, bulbs and seeds for the keen gardener obviously. Everything you'd want that can grow in the British Isles, you will find here. You will also find organic sweets and jams that are very nice. And of course, all the more standard souvenirs, such as key rings et al.
What to buy: Seeds for the keen gardener, sweets for the kids.