Mongo zoned sharding with a single value

We have a database with around 50 collections, and 70 million records. All records belong to specific customers and have a customerId property. Currently all clients are in the USA. However, we're in the process of adding EU clients and would like to host their data in an EU datacenter. Since all records have a customerId (which is a string) and virtually all of our queries also specify this customerId this makes it a good choice for a shard key for us - we're only going to have one shard per zone, and we'd like all of a customers data to be in that one shard. My question is, given that the customerId's are strings, how do we specify a minimum and maximum for the sh.updateZoneKeyRange() function? Obiously minimum will be the customerId (e.g., "customer-somename") but how do we specify a maximum of "customer-somename" + 1? The issue here is that maximum is exclusive, so it can't be the same as minimum