greens can be frozen, but during the thawing process, they tend to turn to mush or become soft. ALso, freezing does destroy certain nutrients as well, Thalimine is one, I beleive. Best way to cut down waste, start eating the greens yourself
Most good healthy greens for blue tongue skinks, green iguanas, bearded dragons and other vegetarian reptiles/mammals are good tasting and very healthy for us. Collard greens can be cooked, (no clue what kind of receipies are available for them, but I suspect you can cook them like you would spinach), endive, dandelion, escaroli, etc. are great as a mixed salad. Some are a little tart but when all mixed up in a salad with a good salad dressing it tastes really good.
You can also wash and dry each green well, then place in single layers on papertowel and store in the fridge. This deffinitely prolongs the life of the greens. Or, if you know people who have vegetarian animals (guinea pigs, rabbits, bearded dragons, uromastyx etc) offer to split the green cost with them. They may have problems with left overs going bad so paying half the cost of the greens and splitting them between eachother would be beneficial to both of you.