The normal way to do this would be to create a table containing your services and their associated nominal codes. Then, build your value list from the service (using the contents of the field). You can then use either a related field (using the service as the key field between the tables) or a lookup to show the nominal code.
Table - Services
One Way 4000 Return 4001 ALG 4002 CS 4004 CML 4006 INSURANCE 4007 scooters 4008
Each service is a record (row) in your utility table.
By all means ...