I've never used UART sanded paper, so can't speak from experience about that brand. I may often spray my work during its production, but the last layer or two I may leave unfixed. Lately I've been spraying with Spectrafix (Casein formula), rather than the usual fixatives that you get from W&N, Daler, etc; it seems to dry back fine and I haven't noticed any negative effects. One thing to note with Spectrafix, though, it doesn't spray huge clouds of fine "mist" like other dispensers (well, mine doesn't, anyway!).
The only way I've managed to get a light touch with a fine spray is to put the work vertically inside a fairly big cardboard box and hold the can at least twelve to fifteen inches from the work's surface.
I send unframed pastels from the U.K; they are lightly fixed. I protect the surface with glassine paper (which prevents static, this can lift pastel particles), before sandwiching the work between two pieces of rigid hardboard, firmly taped left-to-right and top-to-bottom, to prevent slippage and rubbing. This all then goes in a large padded mailbag. If your work isn't fixed, you could attract your buyer's attention to this fact in red-coloured type in your dispatch letter. And maybe create a large label or card with the same info, that the buyer can take with them to the framer's. Plus, encourage them to retain the protective papers and board.
P.S I've found that almost all pastel papers and cards need a little fixative (have to be careful with Sennelier-card!). The closest to a zero-fix is PastelMat.
Sorry for the ramble....just love pastels and papers!