Attribution from keywords

Site URL: https://bootblackbrand.com

I'm doing some SEO work for a client on this site.  When I end the relationship I want to be able to give him the tools to track things without me.

How can we gauge attribution with the existing Squarespace tools?  Or is it possible?

Attributing sales to 'Search' isn't going to cut it because most of the searches are branded.  We can see increased traffic from certain keywords, and increased traffic overall, but can we attribute sales to organic search strings? 

Obviously we can use other tools ,what I'm hoping for is a Squarespace solution

Unfortunately I don't think this is possible just with Squarespace Analytics. You'd need a third-party tool like Google Analytics (and it sounds like you're aware of what the setup there would look like already). I've worked around this in the past by setting up GA and ecommerce tracking, and then sharing a link to a Google Data Studio dashboard with the client which can be a pretty self-service way of looking at the data.