I think you should work on your websites user interface. It is lacking and rather difficult to navigate.
I agree. Look at other puzzles stores like HKNowStore, TheCubicle, ect. and see how their sites are laid out. I'm sure you can even find website templates online that will fit your needs if you don't wish to build up the UI from scratch. As for promotional items, I think cube bags, puzzle stands, logo stickers. Try to stick to puzzle related items for a start. If you website gains more popularity then you can possible move on to other items. Unless you are giving them away or something, I can't see that shirts, hats, and such would sell very well with the current consumer base you have. You puzzles are a good price although you seem to be out of stock of quite a few puzzles I'd like to purchase.