Reporting back- Yep, the topper that was on the display is the one that's part of the sale. I still really like it, DS loved it, and DH has even decided that it's pretty darn comfortable.
I'll be at Ikea with bells on Saturday morning.
And just in case anyone else was curious about the box spring fitting in the Ikea bed issue... of the 3 beds I looked at, all of them would accommodate either the box spring, OR the slatted platform. There were some beds that were very low to the ground that would not work, but none of them fit the look I was going for anyway.