Can't customise Sunday check-in message

Problem/Motivation

The "Sunday Check-in" price alterator produces a fixed message

Sunday check in costs more. Checking in on Monday instead is much cheaper!

We need sometimes to improve that with a custom message

Steps to reproduce

* enable "Sunday ckeck-in" Price alterator
* start a reservation with check-in on Sunday
* check message on /cart pages

Proposed resolution

Customise or at least let admin disable that message