I always use USPS. The prices aren't astronomical, and I've never had a problem with anything getting lost or delayed at customs. However, if you're shipping an item with a smaller value, you might not be able to add insurance. The last time I tried to insure an item I sent overseas (it was just a t-shirt from Victoria's Secret), the minimum I could have paid was $25, which cost more than the shirt itself. I don't have to ship internationally very often, but I don't seem to remember that being an issue in the past, so maybe it's a newer policy.
I also seem to recall hearing that if you ship internationally with UPS, they tack on a lot of extra fees and charges that you won't get with USPS.