Dont try to convince your client, try to find the justifications and analysis that using BizTalk would add value or not.
If your client requirement is just to do integration for this
CSV to EDI, then I would not invest money and resource for any new integration product at all. Try to find the existing product/technology which
would satisfy this requirement.
Analyse the volume of integration, Are there any chances for the volume to increase over the time? Analyse how the existing product/technology can handle this predicted volume and analysis the existing product
based its scalability capability.
Compare the cost and time involved to implement this requirement with any of existing product/technology against BizTalk (or any new integration product)
Analyse whether your client has skillset to handle this requirement with any of the existing product. See how much effort/time/cost requirement to acquire the new skill set and retain the skill.
When it comes to BizTalk, with more than 12,000 installed customers, including 81 percent of the Fortune Global 100 and excellent customer support, BizTalk is the leading integration product in market
mean to handle this requirement effectively.
But the right process is to analyse the variable capabilities of BizTalk and other integration products/ technologies to choose the right solution for your client.
There are various comparison factors you have to look into before choosing any product. Forcing BizTalk (or any product) would not going to satisfy your client in long-time.