A lot of the eBay stores will only use USPS for international shipping because they (eBay) have partnered with them.
eg.
http://ebaysupplies.usps.com/usps
I have heard that eBay also takes less of a cut from the selling amount if they use them.
I have approached stores offering to pay the difference in shipping, just to use UPS or FedEx. Their response to me was no. Accept that USPS will deliver or go somewhere else.
As Ive already mentioned in another thread, USPS mysteriously "lost" my LP I had paid 17k for (purchased from a Gold rated seller). And I got this mailed to me after about 10 calls to US customs, USPS as well as the store that sold it, EMS, JIMC EMS, and a contact inside the SAPO:
Date: June 28, 2013 Thomas Nolan:
In response to your request dated June 24, 2013, we regret to inform you that we were unable to locate any delivery information in our records regarding your item number EC*************US.
If you would like additional assistance with this inquiry, please take this receipt to your local Post Office or postal representative.
Sincerely,
United States Postal Service
This after their tracking site held record of its movements up until the JFK airport hanger. And having paid for an expedited delivery fee of 2.7k, and about 8 weeks of waiting. I have it on record from our side that this parcel never landed here, and mysteriously no one at USPS on the phone could even tell us on what plane, going to where it was supposedly loaded. Then after I bitch to them about it, I get a letter saying they have no record of it, even though they told me on the phone a few days earlier it was on its way to me.
I have in the past bought items and had them shipped with UPS and FedEx, and all those times it was an absolute pleasure, but with the current, "we only use USPS" agenda, I will never buy from eBay again from a vendor that insists on using them.
Here's some proof of the goings on at JFK airport (read the comments below the article):
http://www.pissedconsumer.com/reviews-by-company/usps/content-of-the-package-stolen-replaced-in-isc-new-york-20110331229862.html