Mongo zoned sharding with a single value
Assuming your customerId does not have any obvious mapping to location, it would be best to add a location field and create a compound shard key. The location field should be a prefix of the shard key, otherwise you will have an administrative headache trying to maintain zones for small ranges of customerID values (which could end up as granular as a single customer).

With a compound shard key of  {location: 1, customerId:1} your zone ranges would be straightforward to define.

For example:

sh.updateZoneKeyRange(
  "mydb.mycollection",
  { "location" : "US", "customerId" : MinKey },
  { "location" : "US", "customerId" : MaxKey },
  "US"
)


The MongoDB manual includes a similar example in Segmenting Data by Location and this is the same approach used for MongoDB Atlas' Global Clusters feature. Both of these examples suggest two-character country codes (ISO 3166-1 Alpha-2) for the location field, but you could choose any values which provide suitable granualarity for your future use cases. For example, ISO 3166-2 would provide for countries and subdivisions (administrative regions like state or province).

Starting in MongoDB 4.2, you can also change a document's shard key value if you need to move existing user data between zones. You presumably would never want to change your customerId values, but could update a location field.