Don't always try to take petty advantages of others.
Tips
usage
The pián reading is almost a closed set: 便宜 (cheap / a petty advantage), 便宜货 (a bargain), 占便宜 (to take advantage), and the literary 便便 (pot-bellied). Everywhere else 便 is biàn.
mistakes
Beginners often say 便宜 as biànyí - wrong. The cheap-sense is always piányi (second syllable toneless in everyday speech). The same characters read biànyí only in the rare set sense 'as one sees fit / suit oneself' (e.g. 便宜行事), where 便 reverts to its 'convenience' meaning.
Left person radical - the side-form of 人, a standing figure. The indexing radical (Kangxi #9). 便 is about people making things easier for themselves, so the human focus is literal. Same radical heads 你, 他, 们, 住 - the bulk of person-related vocab.
Right component 更 - historically supplies the sound (gēng → biàn, with heavy Old Chinese drift). It also carries a faint semantic echo: changing things to suit a person = adapting for convenience. The 便/便 split inside this character reflects two old branches of the same word.
