You can set the shipping rates and options on each price sheet.
Go to Commerce > Pricing and then click on the individual price sheet you'd like to work on.
The Shipping area located on the far right-hand side of the page will control the shipping options on orders placed using this price sheet.
On A Lab Fulfilled Price Sheet:
You can activate the option to let your clients pay the same shipping fee the lab will charge you when drop shipping the order. Deactivating the lab shipping option will allow you to create your own custom flat rate shipping option(s).
On A Self-fulfilled Price Sheet:
If you are working on a self-fulfilled price sheet, you will simply set your own flat rate shipping options in this same Shipping section.