Populating a Custom Field via a Plan Choice in Charge?

Question

We are using Charge and have two custom fields we want to populate when a charge is successful. One works and one doesn't.

We can get it to populate a "pageTitle" field which is pretty standard as a hidden field.

The other field's value needs to change based on the selected planChoice - this one doesn't save anything. The Charge works but the custom field remains blank. Any ideas how to get this to work?

Here's stripped down code:

<form ...>
    ... charge specific default stuff ...

    <input type="hidden" name="fields[pageTitle]" value="{{ entry.title | e }}"/><!-- This is a custom field on the Charge, that does get saved -->

    {% set options = {
        'planChoices' : {
            'two' : {
                'label' : "This is a standard Charge field #{entry.title}",
                'planAmount' : (ticketPrice * 2),
                'fields[ticketInfo]' : "This is a custom field on Charge that doesn't get saved #{entry.title}"
             },
             ...
         }
    } %}

    {{ craft.charge.setPaymentOptions(options) }}

    {{ forms.select({
        label:   'Plan Choice',
        name:    'planChoice',
        id:      'planChoice',
        options: options.planChoices,
        value:   (charge is defined ? charge.planChoice),
        errors:  (charge is defined ? charge.getErrors('planChoice'))
    }) }}

    ... other normal form bits ...
</form>