To use a 40ft recycled shipping container or go for modular flatpak?
The space will be used as a retail showroom, mini cafe and a space to rent for events - yoga workshops, meditation etc.
What does your marketing research tell you?
In addition to the cost of the unit (no idea what a flatpak is), you need to consider how stable the unit is during transport, how easy it is to set it up, how easy it is to move, how difficult it is to store, what regulations govern temporary installation of such structures, the cost to customize, the visual appeal once it is in place, temperature and other comfort controls of working in the space, and what impression it makes on users/visitors overall.
Frankly a structured space like that sounds too complicated to me. There are very easy other temporary options and you'll have a tall order to persuade people to consider your structured space.
Additionally, with the pandemic it's a terrible time to be considering anything that is located in small spaces. If you do any research, I think you'll find no interest right now from potential users, and probably not for a couple years.