Multiple products on add to cart? E.g. node contains a product Poster for $5 and e.g. a checkbox saying "+ Cup" which is another $5. So it's two different products, how do I let user select if they want just a poster of if they want the poster + the cup, so check the box for the cup and on add to cart it adds both items?

If you create a product reference field with Unlimited, it won't let to choose multiple items, it renders as a dropbox/select list for one item to choose and add to cart.

https://www.drupal.org/project/commerce_product_kit is locked to total products set for the page in the product reference field, so it always adds both products on Add to cart, no select option.

So basically how to add 1 to 2 products on same page clicking Add to cart only once.
Have you already tried the Commerce Product Add-On module? That sounds like exactly what you're describing.