I know this smacks of return fraud, but depending on how you do this, it can involve shoplifting, too. And yeah, I know I'm not reinventing the wheel, but some newbies might find this useful.
Let's say you want some lotion, but it's crazy expensive, and the bottle is so big that you don't want to risk getting caught smuggling it out of the store.
Find something similar at a dollar store. You can either pay the dollar, or else steal it from there, since dollar stores are generally easier to hit.
Go back and buy the expensive lotion, making sure you get it from a store that has a good return policy. Be sure to keep the receipt.
Put the expensive lotion into an empty container you've saved for this purpose. One way is to put it in a ziplock bag. Then you can cut off a little corner of the bag, and squeeze the product into the empty container without spilling much of it.
Now, take the dollar store crap and put it back into the expensive lotion bottle until it's as full as it was when you bought it. Make sure you do a neat job of this, so as not to arouse suspicion.
Return the bottle with your receipt. You've just gotten some expensive lotion for a dollar or less.
You can do something similar with things like pesticides. If the dollar store has nothing you can replace it with, then just use 2/3 of the bottle and refill the rest with water. It will still smell like full-strength pesticide, but you've just sprayed your home for free.
Use your imagination, but be sure to do this at different stores so that nobody notices a pattern.