I have seen mystery figures for sale in a shops which don't normally sell PM (e.g. Asda, a supermarket), so perhaps the marketing strategy is to offer cases for sale to such retailers, in the hopes of getting a toehold into a larger market for PM sets? Anyway, the PM web shop only offers them on a similar basis to how they are available in retail shops; pot luck which figure you get.
If you can find a sympathetic toy shop owner (first problem, find a toy shop!) they might be willing to order you a case of mystery figures. When I did this for Series 4 (a long time ago!), I got 4 of each figure for slightly less than the retail price each.