how can I add a popup to show a coupon discount code if cart total is high enough?

I need to popup or display specific checkout discount code to people who spend over xxx amount into their carts

I don't want to advertise these codes, just display them to the people who have enough items in their cart to qualify.

I thought to do a check within the cart sidebox itself, checking the ..

PHP Code:

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$_SESSION['cart']->show_total() 


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So that when it is larger a specific amount, display a small image or link in the bottom of the cart that can display the code, or maybe this should be a popup, but then it might get blocked.

Anyone any ideas on the best way to impliment this..

Regards

Steven