Amplify Snack Brands Inc.’s shares are down 8% in their IPO debut. Bringing the market cap of the maker of SkinnyPop popcorn and Paqui tortilla chips at $1.24 billion. The firm’s IPO raised $270 million, after its shares sold for $18 each. Shareholders sold all 15 million shares in the IPO, the majority of which were offered by TA Associates, a private equity firm based in Boston, which owns a 58.3% stake in the company.