stick framed floors are quite common under timberframes
just frame the mortises for your post-bottom tenons in the right spots and save your money, use it to buy a nice chisel!
alternately make no tenons on the post-bottoms and use big screws to attach the posts to the floor deck and framing.
btw i don't think shingles do well on a flat surface, you may need furring strips.
-M.
