See, in fact you have 3 clear sets of content+actions, thus this makes it easier to uinderstand how to place elements within those sets.
1st set: Display
In this set, you present the product to the user. You may have different content elements (photo, name, price, maybe a short description), but it basically works as a teaser to go to the next step.
The display set has a subset of actions, althought the most common one is "click" to step 2. This is a pre-sell action and the part where you must pay more attention.
2nd set: Product
Here you have all expanded content and sub-sets of content and actions relative to content itself. This is where you expand info and allow the user to customize the product. ALL action sub-sets related to product go in this step. Thus, you can add the "add to favorites" here, but do it using iconography instead of buttons, and simply switch on/off visual treatment of the icon, don't add any additional noise.
Then, something I don't see in your wireframes: the actual CTA to effectively make the purchase by going to checkout or add to cart. The possible CTA options are "add to cart" and "add to cart and checkout" (don't mind the copy, just the concept)
3rd set: Purchase
Here, you have 2 options:
1) User selects product and adds to cart, then continues checking other content (pending purchase)
In this case, your second step/set is enough, you don't need your 3rd step AT ALL.
2) User adds to cart and checkout (effective purchase)
Here, you add a detail of the product, price, your ETA box, whatever. This is the final step, and you have to make it straight to the point. You want to sell, and nothing else. Do NOT add elements related to product other than maybe a "change" link at the side or a "go back" button, and an amount box (if applicable, in case people needs to change amount of purchased products without cancelling the process and starting again). But size, color and whatever infor about the product are totally out of place. Same goes for the "add to wishlist" button, because you can't add a checkout process to favorites or wishlist, you add a product!.
Thus, if you have your merchant process, you include it here, if you send users to a merchant processing flow (Paypal, processors, etc), then simply have a "buy now" button
See image below for a visual explanation of the above:
In short, try to always think in terms of logical blocks of information, and how can you group elements in sets, and you'll be able to manage the page/app strcuture in a lot easier way