What are the best practices for outsourcing back operations of an e-commerce shop ?

I a solo-preneur on year 7 of running a successful e-commerce company and am completely burned out on running the day to day operations but I don't want to close up shop. I love marketing and organizing the visuals as well as adding to our product mix and merchandising. I am looking for advice on how to outsource the shipping and customer service piece of the business as well as pulling orders from 6+ vendors. Most of our items are custom made per customer so it isn't suited for a pick & pack kind of fulfillment center. I need fulfillment that is tailored to custom items. I really would like to avoid hiring an in-house employee if at all possible! Thanks in advance!