The United Nations Centre for Trade Facilitation and eBusiness (UN/CEFACT) provides the Core Components approach for defining standard business document types based on all-embracing, reusable building blocks. For utilizing standard business document types in concrete business scenarios, these standard business document types need customizing resulting in business document type variants. However, the current approach is missing sufficient mechanisms for managing the resulting variants. First, customizing standard business document types is currently based on a textual specification only. Hence, variability within business document types is missing an explicit representation. Second, supporting the process of customizing standard business document types requires dedicated tool-support. In this paper, we formalize variability within the Core Components approach by adopting concepts from variability management and introduce a declarative approach based on variability models for validating the process of business document type customization. Furthermore, we present an evaluation of the approach proposed by means of a feasibility study. The approach provides a generic method for validating customized standard business document types.