The Pack Shop can be found under the Shop menu (labelled 特別商品 when in Japanese).
Special packs can be purchased from the Pack Shop, which typically contain Ability Stones (Paid and Unpaid), Scout Tickets, Power Up Materials, and/or other useful items.
"Regular / 通常商品" packs are available always (but can still only be purchased a certain maximum number of times), whereas "Limited / 期間商品" packs are only available during a limited time period.
The Pack Shop was introduced to Bungo Mayoi (JP) on December 27, 2018[1] and to Bungo Tales (EN) on February 5, 2019 [2].