Troy,
Check your code for the page, there are several requests for images which do not exist (namely images/img_tal1.gif, tal2.gif, tal3.gif, tal4.gif, and some others), and these return a 404 page which is not https.
I don't see any other issues though offhand, so fix those bad requests and see what happens then. I'm sure it is not a certificate issue though.
Just having links to http pages wouldn't cause the error (but I imagine at some point you'll config OSCommerce to use all https).
Wayne Reavill
WebHostingBuzz CIO