Customers pay for empty legs all the time. They are not 135, they are not 121. They are 91.

A trip could be quoted:

CLT-AVL-ATL-CLT at rate x, or
CLT-AVL-ATL at higher rate y.

The end price for the charter is about the same. Are they paying for the empty leg on both quotes?

Yes, it counts against your monthly limits as a pilot.

Step 1)
Check your A001 Opspec. Make sure it says you can do part 91 repositioning, training flights, etc.

Step 2)
Look in the back.

Got freight? yes = 135 no = 91.

Now get that flight plan on file - doesn't matter what the wx says - you're going anyway!

Good luck.