At each 7-ELEVEN store (excluding some stores), there is a campaign in which you will receive a "Gudetama charm" when you purchase the target product.
Gudetama charm is being presented at 7-ELEVEN! (Source: 7-ELEVEN official website)
If you purchase two of the target confectionery products, you will receive a charm with "Gudetama" in the image of Halloween. It seems that you should bring the charm to the cashier when you purchase the product. It will end as soon as all 4 types of each store are sold out, limited to 40 pieces.
Halloween-specific Gudetama is cute (Source: 7-ELEVEN official website)
Target products include Meiji "Garbo Mini" (132 yen including tax), Lotte "Pai no Mi" (163 yen), Morinaga "Darth Milk" (119 yen).