Typically I've had my film returned via UPS as it gets to my office overnight and I'm usually mixing sheets and rolls. They've charged me $12 for shipping and packaging and I know the ups portion is less than $5 between DEN and COS. But I've never had a single lab error from them.
In comparison another lab I've used - Slideprinter charges nothing for shipping on processing orders over $50 and $4.95 on orders under $50. I've used them once and had a minor problem with the roll film, but the sheet film all came back perfect. Slideprinter is also all dip-n-dunk for E-6 and C-41.
Slideprinter is also $.50 cheaper per sheet E6 or C41 and a bit cheaper per roll.