[Theseus/production] NoMethodError: undefined method 'us_postage_type' for an instance of Letter::Batch

## Backtrace

    line 232 of [PROJECT_ROOT]/app/controllers/letter/batches_controller.rb: Letter::BatchesController#validate_postage_types
    line 40 of [PROJECT_ROOT]/app/controllers/letter/batches_controller.rb: Letter::BatchesController#update

[View full backtrace and more info at honeybadger.io](https://app.honeybadger.io/projects/132042/faults/121009093)