Well, Your search is over!
This is a double edged sword, but here goes.
I found this very mod at CartMods. Baught it and it didn't work right. Alex stated he would be happy to fix it for me, but I am currently letting others do some other custom work for me and do not want to two fighting back and forth with file swaps at the same time and have opted to contract him later to get it fixed in vs 4.0.9 soon to be 4.0.10.
I do feel he will make sure it works, but I wanted to let you know up front there may be issues with the mod and current versions so it may need tweaking prior to running right.
This mod places items in the checkout pages to allow impulse purchases if you will. Customer never leaves checkout and live update of totals, taxes and shipping takes place when item is added to cart right in checkout process. Or so it is described to do this.
Anyway, here is the link, check it out and let me know how it goes for you. I hope to soon be able to get mine working with his help as soon as other issues have been completed.
http://www.cartmods.com/product.php?productid=60&cat=1&page=1