Big Bazaar — only part of the order delivered

Address: Chennai, Tamil Nadu

Order no BBMB[protected] valued ₹1040.00 (6 items), placed 11-8-18 on BigBazaar online site. After 15 days, with invoice no KA[protected], they delivered only 2 items valued ₹357.76. After regular followups, they called 3 days back and said that for some reason they can't deliver the remaining, instead will share e-voucher worth ₹442.00. I suppose to get ₹682.[protected]=682) back. The same explained customer care through phone and mail with proper order form. Today they called and repeated the e-voucher story again (value ₹442). Saying the order has only for 3 items. Please help me.