For cheapest shocking, get a large container of non-chlorine shock. They all work about the same - MPS is MPS. We like the "Freshwater Oxidizer" which is made by the same folks that make Leisure Time - private labled for HotSpring but not very expensive. Be sure to keep it dry - maybe even break it into smaller containers.
Here in SoCal, I can sell several other brands of MPS for $5 or $6 per bottle less than 'LT Renew.' I don't know why that and OZ are so high, but I haven't had them on the shelf in years.
But be sure you don't go to a pool store or the pool isle of a home center and end up with Clacium Hypo Chlorite, which is labled as 'shock' by some companies. It IS a shock, suitable for vinyl pools, but is not chlorine-free, and has a very high Ph.
I don't see why you couldn't buy the N2 cartridge and put it in or on the filter in a Sundance - many of our customers do.