I think it needs to be what you want your shop to be about. I could ask this same exact question....I sell my actual blankets, but I also sell patterns of those blankets. The patterns sell way more than the actual items, but the actual items get more views. However, I want my shop to be about my actual blankets....the patterns are an afterthought, so I would never focus on that part of my shop, even if the sales mostly come from there. If you are wanting many sales and a good amount of profit, I would sale the postcards. If you want your shop to be about your original paintings, I would say put more of those. :)