We have an old dog, and he gets a goopy food additive to help with arthritis. By looking around online, I've also found it for half the normal price (though not consistently). In fact, the cheapest I've ever found it I'm pretty sure was through the site you linked. It's the 4cyte liquid stuff, and I got a 4 pack for less than $200 - but only the one time.
For fleas, we use a seresto collar. It lasts for about 8 months, and we put on a fresh one in the spring when the risk is highest. Through the winter we typically don't worry and just leave it on for the full 12 months, but I will often do a cheap supermarket dose of flea treatment halfway through winter just to tide over the full 12 months. I see they have cat ones as well: https://www.myvet.co.nz/seresto-flea-and-tick-collar-for-cats.html
Doing that would probably be significantly cheaper, but now that I think about I think many cats don't like collars and end up getting them off, so maybe that's not going to work. We don't have a cat (despite my kids pleading for a kitten) so I'm not very experienced.