If you have backend dependencies (fields, records, scripts) are the able to be deployed with the extension or do they have to be packaged separately?

You mean non-SuiteCommerce scripts? You can't really bundle them into the extension, but you could bundle them in a bundle (lol) or some other SDF package. SDF has supported extensions for a while, but this approach is more technology solution partners who are selling/distributing a package rather than an individual customer, if that makes sense?

I will have to check out how to deploy through SDF. I use SDF frequently.

OK, well a couple of years ago I wrote a guide about using SDF with SuiteCommerce: https://developers.suitecommerce.com/get-started-with-the-suitecloud-development-framework-sdf-and-suitecommerce.html