[Tifosi]

No reason you couldn't do it yourself if they're just conventional belts.

1 big bolt at the sill to hold the free running end, 1 big bolt at the reel to hold that in place, belt threaded through hoop on seatback. That's it.

You'll need to remove the cargo panel to get at the reels, but otherwise it's about as straight forward as it can get. (be prepared for the anchoring bolts to be tight)

Have you tried removing the cargo shelf, pulling the belts all the way out as far as they'd go, clean things, spritz a little lube (silicone or lithium based) into the reel mechanism to lube the pivots, rollers and springs, then working the belt in and out a few times to spread things around, free stuff up and generally make sure the belt is rolled up straight and true?