NEVER work under a running vehicle is a good rule for doing your own maintenance at home. Once you fill the tranny get in and start the vehicle then start cycling through the gears while holding a firm brake pedal. Stay in each selector position for about thirty seconds going in each direction on the selector. Shut down the vehicle and then check your level, add as needed then repeat.

The shop you had the flush done at should have known that you ALWAYS cycle the tranny before final level check. The most basic of mechanics knows this about auto trannys.