The CallingPost trailer branding is tied to your subscription plan. It's not a feature you can pay to remove independently. Here's a breakdown:
Premium Unlimited Monthly Plans: These plans include the removal of the promotional portion of the CallingPost trailer. Upgrading to Premium Unlimited is the only way to achieve this. The cost of removing the trailer, therefore, is the cost of upgrading to the Premium Unlimited plan.
Essentials Unlimited and Pay As You Go Plans: These plans use the full CallingPost trailer, including the promotional message. There is no option to pay extra to shorten or remove it on these plans. You must upgrade to a Premium Unlimited plan.
In short, there's no separate "reduced trailer feature" you can purchase. The level of branding is directly tied to your chosen subscription tier. Upgrading to Premium Unlimited is the only way to remove the promotional message.