Is there any way to have this as an option for customers and if so would it also adjust the shipping rate? Right now I just make all FedEx signature service but many don't want that so I wonder what the solution would be.
Signature can be provided in the Multi Carrier Shipping label app. You can request the team to enable it for your account. Once done, an automation rule can be setup that would allow signature based on a variety of factors as per your requirement.