Ok, to play devil's advocate here.
1. Don't do it at an indirect. Best buy may be ok because they're large, and probably wouldn't notice/care enough to come after you with a chargeback. At my indirect you sign a separate contract saying if you cancel within 6 months, we will bill you full retail on the phone or you must return it to us.
2. If you cancel within the first 14 days, YES, corporate can bill your account full retail on the phone. Most carriers have a "gaming" department that handles cases like these.
(Source: I used to work in a gaming dept. Charged a guy full retail on 10 blackberry bolds
3. You'll have the cost at the POS, a $35 activation fee, half a month of service, and the ETF. Just be aware of all of these.
4. When you call in to cancel DO NOT say it was lost/stolen. Don't want your ESN getting flagged