combining drupal commerce product display page with checkout page I am trying to find a good way of combining the Drupal Commerce Product Display page with the checkout page. I am trying to eliminate a page, so that the user would choose the product variation and fill in his profile data all on the same page.

I see that there is a hook_commerce_checkout_page_info_alter() which looks like it may be a start, but it would need more thought of how to actually combine these two pages.
You can't get to checkout pages without products in your cart ( try visiting /checkout with an empty cart). At least, not without extensive hacks.

I'd recommend keeping the catalogue browse / add to cart separate from the checkout process.

If suitable, you can redirect straight to checkout with a Rule after the product is added to the cart, for user detail input and order completion.