Did you read terms and conditions for the voucher before buying? I remember planning to buy vouchers for buying lenses for my sister. Vouchers explicitly mentioned the branded lenses exclusion, hence I didn’t buy then.
I just nowndid a search and found American express lenskart vouchers. In there it is explicitly mentioned:
“7GV/GC cannot be used for branded contact lenses, including but not limited to Bausch + Lomb, Johnson & Johnson, Alcon, Freshlook, and Acuvue. Excluded brands may be updated by Lenskart from time to time.
8.GV/GC can only be used for Aqualens contact lenses on Lenskart’s website, app, offline stores, and affiliate platforms.”