I almost always use 3rd party sellers on Amazon, so I don't think I've gotten stuck with OnTrac. The last time I ordered from Amazon directly, it came USPS direct.
Though, I have had Amazon direct stuff come UPS. I wish those assholes would have multiple choices for shipping on everything.
On ebay, if I have a choice of sellers, I always opt for the one that promises USPS instead of "standard shipping", which usually means a combination of either UPS Store/USPS or DHL/USPS, both of which take at least twice as long as just USPS.