Yeah - I should qualify my last statement: Use USPS if you can.
Sadly, many US retailers don't send via parcel post, so, if you are forced to use courier, look at the air vs ground rates. At a bare minimum the customs "handling" charges are about $30, and go up with the value of the item. If the air shipping is only $50 more, it's probably a good idea to go that direction, as all of these BS charges are included with air delivery.
Some other things I've discovered when using couriers for US purchases:
UPS will also charge you a service fee for paying the customs and collecting it from you at the door. If you phone them and give them a credit card to pre-pay, you can usually avoid 7 or so bucks of charges. (Not to mention that the driver can then drop the package off without someone being there, if you want them to, that is). ... or in my case, leave $2000 worth of electronics on my neighbours doorstep, in the rain.
Bulk order if you can. An $80 handling charge on a $800 order is easier to stomach than a $30 handling charge on a $50 item.
Fed-Ex isn't much better. They charge pretty much the same fees, but send you an invoice in the mail, two weeks after you get the package.
Also, if dealing with a retailer and purchasing multiple items, make sure they send them all as one shipment! Otherwise you will get charged for multiple handling fees.
You can send it back and not pay for the handling charge. I've done this a couple times where I explicitly told the seller to send me via parcel post and it showed up in a UPS van.
Supposedly, you can also "Self Clear" your packages. It involves driving out to main UPS depot, picking up some paperwork, driving over to the customs building (also near the airport) and back. Probably takes about 30-60 minutes to do depending on traffic and lineups. I say "Supposedly" because I tried to do this with a Fed-Ex delivery and they told me that the parcel had to be cleared in Vancouver because that is where it entered the country.
The really criminal thing is not that they charge these handling fees, but that they are hidden until the driver shows up with the bill. This stuff should be included in the up-front shipping charges. Retaillers use the couriers exclusively because they think the service is better and cheaper than parcel post. (Which is probably true in the States). There's a lot of stuff that wouldn't be ordered if people knew what it was going to cost them.