It really depends. If you are having CYA issues and it needs adjusting, then the TF-100 kit will give you the best bang for the buck because it comes with more reagent. They also sell that seperately as well, so you could supplement your kit with the FAS-DPD test and the CYA test, if you can't return the one you have.
If CYA levels are stable and you aren't adding any via trichlor/dichlor use, you can test it once or twice a year, if you have a cartridge filter. Sand filters and DE filters are backwashed, so they have to replace that water with fresh water, which causes the CYA level to drop over the season, so those type of filters would require the CYA test more often than a cart filter.
I would not rely on Pool stores for testing CYA - they are often inaccurate or don't even bother to test for it. There was a thread yesterday about a pool store that told him his CYA was 140. Turns out it was zero. Can you imagine if it had been the otherway around and he added CYA when it was really 140???!
Anyway, first preference would be return it, if you can't then order those two tests individually.
Hope this helps.