[shabble]

Having a click-through example.com/deals page which sets a hidden field or drops a cookie could avoid some of the Sad Missed Deal approach.

Then again, it immediately informs your potential customers of all your available deals, without the hassle of searching around, so they're more likely to find some way of optimising their payment downwards. You could (pseudo-)randomly display certain deals, or set rate/quantity caps ("Buy quickly, only 50 coupons remain!") to deal with that.

By providing visibility on all your deals, the customer feels more satisfied knowing they got the best possible deal, and you limit the proliferation of all those annoying voucher search sites (and cart abandonment when none of those 3-year old $5 off codes work).