Ok, since I cannot access your install, things are a little confusing. Why do you have blog posts loading into a coupon "page" that you created? Really you don't need a page to do this, you just create a Coupon category and then when you create posts assign them to that category. And then yoursite.com/coupons will load the posts just from that category. (you'd have to delete the page and empty it out of the trash so that the /coupon slug becomes available)
As far as I know, unless you're using a certain template on your coupon "page" that allows you to choose which category of blog posts to show up on it (I haven't used WordPress default themes in a long time so I do not know if it is possible with them, but other themes like Genesis child themes allow this), then I'm not sure how you're accomplishing that. Can you make sure that the template for the coupon "page" is using the default template?
I do see you have /category/coupons which is loading your /coupon post about Walmart... is that what is supposed to be showing up on the /coupons page?