A number of online stores I shop offer several shipping options, some as many as four, for different prices. Are you just automatically selecting the lowest cost option? I assume then you are charging us the actual shipping cost for the selected option, so some customers will end up paying a little more or a little less total for the same product after shipping is calculated? (or does everyone pay the same estimated rate regardless of how they are shipped to?)
I know I personally would like to be able to pick my carrier if you're going to be flexible about it. I personally have had many bad experiences with USPS, but fedex/ups are about the same. Others here have described ongoing problems with either fedex or ups - it often comes down to the service you get from your endpoint. Some of us just have terrible delivery service from some carriers, and would gladly pay a little more to choose a different carrier.
I assume adding this feature to your website is a little inconvenient, but it'd be appreciated.