Why does 'free' tempt you far more than 'almost free'?
The jump from 1,000 VND to free pulls harder than any other price cut of the same size.
Free is not just the bottom of the price scale - your brain treats it as its own category. A zero price means zero risk of losing money or overpaying, which adds a rush of good feeling on top of the item's real worth. So you over-grab the free thing, even when a paid option is the better deal once you weigh what you actually get.
A shop offers a 20,000 VND voucher for 5,000 VND or a 10,000 VND voucher for free. Most people grab the free one, even though paying 5,000 nets you 15,000 in value versus only 10,000 for free.
Free flips a switch in your head - so before you say yes to it, ask what the paid option actually gives you for the difference.
Free shipping thresholds, free gifts, and free trials all ride this pull; naming what 'free' is really costing you in time or a bigger purchase keeps it from steering you.
Zero is not a number on the scale - it is a magnet.
Learn the idea and practice English at the same time.