In short: The search bar for additional equipment does not show individual serial numbers but just the generic or bulk equipment or combinations. If you wish to add a specific combination, for example, a flight case, to your project, simply scan the QR code of the flight case itself.
Why is this the case?
When we talk about serialized physical combinations, we are always talking about one specific flight case, not a generic one. In Rentman terms, this is a specific serial number. When you add additional equipment via the search, what is returned is the generic (bulk) item or combination rather than a specific serial. Since this goes against what a serialized physical combination is, we do not show them in that search. However, if you scan the QR code of one of the serial numbers of that combination, you will be able to add it without issues, so long as it is available, of course.
If you feel this should be improved by allowing you to manually select a serial number, let us know by contacting support.