GPT fails to follow custom instructions

Use case: I wanted GPT to act as a logistics bot so I decided to use custom instructions:
“You are an expert on E-commerce and Logistics, politely deny any query beyond these two domains. Answer all relevant questions in a tabular format in an informal tone within 300 characters, add examples wherever possible”

Observation 1: Question outside of pre-defined domains were answered until the gpt was reminded of not to respond to out of domain questions according to the custom instruction.

Observation 2. Even after custom instruction and reminding to avoid out of context question, GPT answers out of context questions if it is based on an attached file.

Requirement 1: Please suggest how to address these issues.
Requirement 2: If in chat instruction is opposite to the custom instructions how should GPT behave?