Hi AnointedMatt,
Thanks for the update.
I think it’s very unlikely that the topper you purchased is really 7.5 lb density. Did you happen to weigh it?
The better online options for toppers I’m aware of are listed in post #4 here. Sleep Warehouse is one of the sources there but their 4" Sensus topper is $50 less on Amazon (after shipping) than it is on their own site and I’m not aware of any other sources that would be less.
I would keep in mind that the response time of memory foam has little to do with its quality and some people prefer memory foam with a faster response time and less motion restriction or a “sleeping in sand” feeling (although of course others will prefer slower response times as well).
I would also be somewhat cautious with buying a 4" thick memory foam topper because of the risk or sleeping out of alignment over the course of the night. Memory foam gets softer with temperature, humidity, and the length of time it is continuously compressed and with thicker layers of memory foam there is a risk that you can start off the night in good alignment before it has softened but then wake up in the morning out of alignment (and possibly with a sore lower back).
The instructions that are linked in post #5 here for compressing latex will also work well for compressing memory foam so that you can return it through a courier (hopefully for less than $175)
Phoenix