Please dont ban me for mentioning PopUps

I hate them as much as the next guy.... but:
One of my clients has asked me to consider implementing a PopUp window on his online store whenever a potential customer cancels a purchase whilst they are in the CheckOut stages.
I have warned my client that PopUps are widely considered as being annoying, but he believes he could increase his sales if the PopUp window was used to offer the customer Free Shipping, or a discount.
We have studied the stats of the website for the last month and a majority of Page Exits are made during the CheckOut stages of the site. Since Tax is already included in all prices we can only assume they are scared by the Shipping Fees (which in my opinion are reasonable for the product being offered).
I would appreciate some detailed responses outlining whether or not you think a PopUp (or PopUnder) window should be implemented. I have been thinking about it for a few days now and have the following Pros/Cons already thought out, but would appreciate a few members inputting there opinions, or expanding on what I have already thought of
Pros :
Could increase sales if used effectively
Can offer customer a good discount
Most people use popup blockers so wouldnt be effected anyway
Prevents my client having to do a "site-wide" sale for all customers, only those who decided not to buy
Cons :
PopUps Generally seen as annoying
Could prevent people returning to the store
Would have to store a cookie ensuring it only popped up once
Informs customer that my client can afford to lower his prices but chooses not to, unless he is missing out on a sale.
Finally, let me put it this way....
If you had left an online store due to high shipping costs would you be enticed back into making the purchase if presented with the offer of Free Shipping in a PopUp window?
Many thanks and sorry that this message was so long

I look forward to the grilling :P