Yeah it's okay not to be too fussy about brands. Once a brand makes it big, they bump up the price and u are paying for that name when u can get the same quality for much less.
That being said, buying knock-offs may not be a good idea. As my friend pointed out in a moment that changed my mind completely, when u buy those, you keep your eyes open under the shade of these cheap tints and end up letting more of the harmful rays into your eyes. So be careful! Investing in good certified UV protection is the way to go