choosing paypal integrated plugin

i want to install a paypal integrated plugin on my site that does the following:

1. offer seamless checkout so that the customer doesn’t have to exit the site when making a purchase

2. offers a way to provide bulk discounts if a customer orders more than X units, so the price drops from Y to Z

3. offer coupon codes functionality

any suggestions on which plugin is better and will accomplish these goals??? please advise. thanks in advance.