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Amazon now seems to be offering two free Prime shipping options for orders shipped by Amazon.com. Earlier I could only remember seeing one free option and that was mostly shipped using the horrible i-parcel service. My recent order using the faster of the two free shipping options was shipped using DHL. Hopefully, this is not a one-off as the DHL shipping is much faster.
They use several. I think it comes down to where it's shipped from though it may also take into account actual cost, who knows. I've had them come via regular parcel mail, DHL and Fedex.