Not a problem with the questions! We're all here to learn from each other.
The filling (poly batting) inside the cartridges can be re-used. Just rinse the filling under tap water until they are white again (after removing it from the plastic mouth piece). Gently dry with paper towel and let sit till fully dry. You will likely need a pair of tweezers to re-fold the filling and stuff it back into the plastic mouth piece. You can re-use that filler multiple times until it just doesn't seem to want to work as well as it did. It's also a good idea to use the tweezers when removing used filling to avoid contact with your skin.
People are experimenting with other fillers so that they can use the same plastic mouth pieces again and again. There are lots of opinions of the best cartridge filler. If you do a search for tea bags, coffee filters, fluval, fish filter on here you will find lots of discussion on this.
You mentioned refills for $4/10. That's not bad imo. If they are for your model then you could purchase those knowing they'll fit, and you can get many uses out of each or more if you find alternate fillers to your satisfaction.
Filling your own carts with your own liquid is much more economical than purchasing the pre-filled ones and from what most people say tastes a lot better too. I agree as I continue to branch out into new flavors.
Edit: When you investigate the DIY recipes you will likely note that sometimes distilled water is added for the right consistency. It shouldn't be too thick or too thin which is why I recommend checking out the proportions others have used successfully. While you are waiting for your orders you may want to find yourself an eye drop bottle or something similar. You can add your ingredients into the bottle, shake well, and use that for topping off your cartridges. Some vendors that sell liquids also sell bottles, fer cheap. Additonally if you buy pre-mixed liquids, which is also economical compared to pre-filled carts, many vendors liquids come in squeeze bottles.
I really hope I've not confused?!