UPS charges way less for SurePost and those packages are pretty low on the priority list. I would be surprised if Amazon sent any Prime user (i.e. someone with free two day shipping) a package through SurePost.
I'm pretty sure the delivery address is the #1 factor in choosing the shipping carrier, followed by the fulfillment center. In particular, if there is a fulfillment and/or Amazon sorting center in your metro area, I think Amazon prefers to do all or part of the shipping in-house (either via Amazon Logistics or their linehaul network/Prime Air with the last mile delivered by USPS, the latter which is what we get for most of our orders).
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