Do I need to develop an app for a priceRule change in checkout

[Jamessy]

Hi all

New to Shopify, so excuse my ignorance.

I need to change the checkout total, depending on the results of a third party API call. It looks like the Admin API  PriceRule will do the business. From a dev point of view, is it best to create an app to do this, or is there a more leaner, cleaner method? 

Thanks

James