For example the Swiss one includes this:

The Confederation shall ensure the adequate, universal and reasonably priced provision of postal and telecommunications services in all regions of the country.

So similarly it could say:

The Confederation shall ensure the adequate, universal and reasonable availability and acceptance provision of cash in all regions of the country.

Or let's look at this one:

[The Cantons] shall ensure the provision of an adequate basic education that is available to all children. Basic education is mandatory and is managed or supervised by the state.

Which could similarly read:

[The Cantons] shall ensure the availability of adequate forms of cash that is available to everyone. Acceptance of cash is mandatory and is supervised by the state.

There are also several articles where the Confederation shall encourage things (e.g. education in music and sport) or promote things (e.g. research) so it could also say to encourage/promote usage/acceptance of cash.

Anyway, we can write into our constitutions pretty much whatever we like. Unlike in the US where the constitution is considered untouchable, we essentially treat it like just another law book.